Raise LoadError for unresolved require's

Prior to this change, RubyMotion's toolchain would report the error like so:

    ERROR! Can't resolve dependency `app/file3.rb'

With this change, the error message is more helpful:

    ERROR! In `./app/file1.rb', could not require `file3', file not found.

However, with this change, a stack trace is printed in the later case while
none is printed in the former.