Crash if a config file isn't there.

If a configuration file is specified via the command line but does not exist,
fail loudly and crash to preserve the current state. Do not fail if the requested
file is the default configuration file, _config.yml. In that case, fall back on
the defaults.
Throw ArgumentError when there's an invalid config file, and catch only that.
Raise LoadError if the file doesn't exist but do not catch it. :)
Merge pull request #1098 from mojombo/catch-missing-configs

Crash if a specified config file isn't there