Fail fast and show an error if no spec files are found

[MarkVillacampa]

If there are no spec files, when running rake spec, the app will crash at runtime with this error message:

2014-05-12 19:06:02.608 App[69351:70b] *** Terminating app due to uncaught exception 'NoMethodError', reason: 'spec.rb:476:in `current_context': undefined method `[]' for nil:NilClass (NoMethodError)

Since new Rubymotion projects come with a main_spec.rb sample spec, this will not be a common problem. However this can be very helpful when using the files env variable to select only some spec files (I run into this all the time).

The previous error will still happen if we have empty spec files. This could be solved by checking at runtime if @contexts is empty in spec.rb. But I'm not sure if this is recommended/necessary.

[MarkVillacampa]

I'm not 100% convinced of the error message. If accepted, message suggestions are welcome.

[jacklynrose]

Good idea, might the best message simply be "No specs found"?

[squidpunch]

This problem also arises if you happen to run rake spec files "blah blah blah" when "blah blah blah" happens to be an incorrect path or filename (due to typo, etc). So in that case it would be nice to tell them what files they tried to run, but were incorrect. I put together a PR for the same thing without seeing this here, but was trying to figure out how to report what files were the problem. This seems to be the perfect place for that.

Unsure if I should change up #208 to match how its done here or just wait on this to be merged since it existed before and solves the problem - but has been out here for almost a year.