fix(firebase_ui_auth): Catch sms code validation

[wer-mathurin]

Description

This allow the user to pass an onError function when an error happen when validating the sms code

Related Issues

#481

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• All unit tests pass (melos run test:unit:all doesn't fail).
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.

[wer-mathurin]

@russellwheatley
just a working example of what need to be accomplish.

But the way I've done it may need refactoring. Affecting a completer more than once does not smell well...
Since I'm not familiar with this package, Who can help me with this?

This is a breaking change since the Completer must return a nullable value.