Fail `generate_strings_file_for_glotpress` when it finds reused keys

[mokagio]

As discussed in wordpress-mobile/WordPress-iOS#19651, genstrings does not throw an error upon reused keys, only a warning:

That is not acceptable for us, because it leads to these easy to make errors going ignored and possibly making it past the code freeze stage.

I think we should be able to run the genstring output through RegEx that can match strings with the format Key "..." used with multiple values. Value "..." kept. Value "..." ignored., collect all the matches, and return them as errors.

Note: I'm aware "..." is not valid, or useful, for the RegEx, just using it to give an idea of the shape of the text to look for.

See also wordpress-mobile/WordPress-iOS#20030.