[firebase_auth] Make auth errors uniform and platform independent

[NoamDev]

It's annoying that for every PlatformException being thrown from firebase auth I have to compare the code twice, once for android and once for ios.

Is it possible to use the same error code in all platforms?

[collinjackson]

I agree that this would be a welcome improvement. I don't have the bandwidth to implement this in the short term but would be happy to review PRs.

[NoamDev]

I've found a list of all error codes (https://docs.google.com/spreadsheets/d/1FDn0rRRYjgXMwc_FIAxO7_cQh69q5VXgQt7Hmvyb1Sg)
in this PR flutter/plugins#775.

[NoamDev]

I'm not sure if I can do that, but anyway, we should have a specification for it.
which error code and message do we want, the android or the ios version.