Changing exception on missing con file to not rely on symfony/filesystem

Currently when the user specifies a non-existent config file at the command line, we throw a FileNotFoundException.

Because this is not a non-dev dependency, this currently throws a php fatal error when phpspec is installed without require-dev libraries, because FileNotFoundException is unrecognised. This was not picked up in CI because the dependency is always present.

This PR changes it to throw a RuntimeException instead.

Fixes #404