print error from config file parsing by default

[tmc]

That providing undefined arguments prints some help and exits but -config (nonexistent) doesn't print anything seems like a bit of a mismatch. I realize CommandLine has ExitOnError on by default but figured I'd open up a discussion. What about an explicit check for the file's presence and performing an f.failf in that case?

[johnrichardrinehart]

I'm with @tmc on this one. I'm happy to issue a PR if it will be considered.