Always throw error if config file is specified

[gamgi]

As mentioned in #498 current behaviour is to silently fail if --config file is not found. This can result in running the server with an unintended configuration without the user knowing about it.

The change in this PR ensures that errors are thrown if the --config file is explicitly specified.

Example behaviour:

$ serve -c nonexistent.json
ERROR: Not able to read /foo/nonexistent.json: ENOENT: no such file or directory, open '/foo/nonexistent.json'

[jakearchibald]

Ugh, yes please, merge this. This caught me out, especially as the config path is resolved against the content path, which is pretty unusual.

[longzheng]

I've also been caught out by this issue leading to a very confusing/frustrating situation, coupled with the fact that the config path is relative to the "served folder" and not the current working directory.

It is extremely subtle that the config was/was not being loaded.