SDK errors should not reside in the "internal" package

Users of the SDK are actually very interested, what sort of error was returned from Firebase. Some errors, for example, require deliberate actions on the SDK user side.

Kindly move the Firebase error info into a publicly accessible package.

The issue names no files, tests, or entry points. Start by locating the internal package's Firebase error definitions and their callers, then determine the public package location and compatibility requirements; done means users can inspect the returned Firebase error information through the SDK's public API.

Written by the indexing model from the issue text.