[Source] Gracefully handle unexpected source structure

[fabiopelosin]

A proper informative message should be provided in the following cases:

• There is an unrecognised directory which should represent a version.
  • Related: ArgumentError - Malformed version number string Scripts CocoaPods#2038
• The podspec could not be found (maybe this is already handled)

A good starting point is the Pod::Source::FileSystemDataProvider#versions method.