Add warnings field to JSON return and warn for long set names

[NateLanza]

The JSON returned by generator.py should include a warnings field that details potential issues with the generated alttxt. The first such issue should be if set names are longer than 15 characters; in this case, alttxt should be generated as normal, but with a string message in warnings stating that the length of the set names may hinder readability of the alttxt.