fix(errors): restore file and line info

When upgrading to version 2.0.0 the errors implementation was changed,
and the "filename" and "line" context was removed from the `toString`
representation. This commit restores the information by extending the
`make-error-cause` method.

Fixes #264.

lib/gulp-uglify-error.js

@@ -1,4 +1,13 @@
 'use strict';
 var makeErrorCause = require('make-error-cause');
 
-module.exports = makeErrorCause('GulpUglifyError');
+var gulpUglifyError = makeErrorCause('GulpUglifyError');
+gulpUglifyError.prototype.toString = function() {
+  var cause = this.cause || {};
+
+  return makeErrorCause.BaseError.prototype.toString.call(this) +
+    (this.fileName ? '\nFile: ' + this.fileName : '') +
+    (cause.line ? '\nLine: ' + cause.line : '');
+};
+
+module.exports = gulpUglifyError;