Document and test error handling

Add tests to make sure ReactFire's error handling is behaving as expected, and docs to show how to handle errors when using ReactFire

Read the existing error-handling behavior and the documentation files named in the issue template: docs/quickstart.md, docs/guide.md, and docs/reference.md. Add tests covering the expected ReactFire error handling, then document how users should handle errors; the work is done when the tests pass and the guidance is clear.

Written by the indexing model from the issue text.