[swift-generator] skip un-generate-able constants

There is a bug that the swift generator cannot handle certain kinds of constants (e.g. maps that have enums for key or value type). This bug will take a bit of restructuring in the generator to fix properly, but some of the users of the generator don't actually need those constants generated at this point, they just need the generator to keep going instead of break and fail to generate any further types. This change does that.
(cherry picked from commit 1edc68db13c77cf65e4dd305abeee563cd35c79c)
Use the right name for field annotation template

This was breaking union generation
(cherry picked from commit bf8c8225fd31b7b30f0bd2358f8413a1aaff2131)
Update nifty dep to 0.13.1