Add possible localization/interpolation specific error and fallback

[williamhjcho]

Allow a fallback/default method to deal with failed localizations (such as returning the key value itself) in the options.

And it might also be interesting to create specialized errors to know why/when/where a localization failed and treat it accordingly. Or maybe not, since depending on the error, it might not be possible to recover anyway.
e.g. interpolation failure due to missing variable could already be handled by fallback (plural + context).

[aksh047]

Hi @williamhjcho,

I’d love to work on this issue! I have experience with dart, and I think I can fix this issue. Could you please assign it to me?

Thanks! 😊