How to use the xmlsec.Error function in xmlsec

def __init__(self, keysMngr=None, _obj=None):
        """
        Creates  element processing context. The caller is
        responsible for destroying returned object by calling destroy method.
        keysMngr : the keys manager.
        Returns  : newly context object or None if an error occurs.
        """
        if _obj != None:
            self._o = _obj
            return
        self._o = xmlsecmod.dsigCtxCreate(keysMngr)
        if self._o is None: raise Error('xmlSecDSigCtxCreate() failed')
    def __isprivate(self, name):

def findKey(self, name, key_info_ctx):
        """
        Lookups key in the keys store.
        name       : the desired key name.
        keyInfoCtx : the  node processing context.
        Returns    : a key or None if key is not found or an error occurs.
        """
        _obj = xmlsecmod.keyStoreFindKey(self, name, key_info_ctx)
        if _obj is None: raise Error('xmlSecKeyStoreFindKey() failed')
        return Key(_obj=_obj)

def __init__(self, _obj=None):
        """
        Creates new keys manager. Caller is responsible for freeing it with
        destroy method.
        Returns : the newly keys manager or None if an error occurs.
        """
	if _obj != None:
            self._o = _obj
            return
        self._o = xmlsecmod.keysMngrCreate()
        if self._o is None: raise Error('xmlSecKeysMngrCreate() failed')
    def __isprivate(self, name):

def __init__(self, size=None, _obj=None):
        """
        Creates and initalizes new memory buffer with given size. Caller is
        responsible for calling destroy method to free the buffer.
        size    : the initial buffer size.
        Returns : the buffer or None if an error occurs.
        """
        if _obj != None:
            self._o = _obj
            return
        self._o = xmlsecmod.bufferCreate(size)
        if self._o is None: raise Error('xmlSecBufferCreate() failed')
    def __isprivate(self, name):

def __init__(self, _obj=None):
        """
        Creates new transform of the id klass. The caller is responsible for
        destroying returned tansform using destroy method.
        id      : the transform id to create.
        Returns : newly created transform or None if an error occurs.
        """
        if _obj != None:
            self._o = _obj
            return
        self._o = xmlsecmod.transformCreate()
        if self._o is None: raise Error('xmlSecTransformCreate() failed')
    def destroy(self):

def addKeyName(self, name=None):
        """
        Adds  node to the  node.
        name    : the key name (optional).
        Returns : the newly created  node or None
        if an error occurs.
        """
        _obj = xmlsecmod.tmplKeyInfoAddKeyName(self, name)
        if _obj is None:
            raise Error('xmlSecTmplKeyInfoAddKeyName() failed')
        return libxml2.xmlNode(_obj=_obj)
    def addKeyValue(self):

def __init__(self, dsigCtx=None, origin=None, _obj=None):
        """
        Creates new  element processing context. Caller is
        responsible for destroying the returned context by calling destroy
        method.
        dsigCtx : the parent  node processing context.
        origin  : the reference origin ( or  node).
        Returns : newly created context or None if an error occurs.
        """
        if _obj != None:
            self._o = _obj
            return
        self._o = xmlsecmod.dsigReferenceCtxCreate(dsigCtx, origin)
        if self._o is None: raise Error('xmlSecDSigReferenceCtxCreate() failed')
    def __isprivate(self, name):

def cryptoAppPkcs12Load(filename, pwd, pwdCallback, pwdCallbackCtx):
    """
    Reads key and all associated certificates from the PKCS12 file.
    For uniformity, call cryptoAppKeyLoad instead of this function.
    Pass in format=xmlsec.KeyDataFormatPkcs12.
    filename       : the PKCS12 key filename.
    pwd            : the PKCS12 file password.
    pwdCallback    : the password callback.
    pwdCallbackCtx : the user context for password callback.
    Returns        : the key or None if an error occurs.
    """
    ret = xmlsecmod.cryptoAppPkcs12Load(filename, pwd,
                                        pwdCallback, pwdCallbackCtx)
    if ret is None: raise Error('xmlSecCryptoAppKeyLoad() failed')
    return Key(_obj=ret)
def cryptoAppKeyCertLoad(key, filename, format):

def getSignMethodNode(self):
        """
        Gets  child of  node.
        Returns :  node or None if an error occurs.
        """
        _obj = xmlsecmod.tmplSignatureGetSignMethodNode(self)
        if _obj is None:
            raise Error('xmlSecTmplSignatureGetSignMethodNode() failed')
        return libxml2.xmlNode(_obj=_obj)
    def getC14NMethodNode(self):

def __init__(self, _obj=None):
        """
        Creates transforms chain processing context. The caller is responsible
        for destroying returned object by calling destroy method.
        Returns : newly context object or None if an error occurs.
        """
        if _obj != None:
            self._o = _obj
            return
        self._o = xmlsecmod.transformCtxCreate()
        if self._o is None: raise Error('xmlSecTransformCtxCreate() failed')
    def destroy(self):