fix(avatars): clean up numerous issues from comments in #1405

This fixes #1462 and various issues raised by @pdehaan @shane-tomlinson and @ckarlof.

@shane-tomlinson r?
Should this be translated? Not sure if these are user-facing errors.
Also, should these be `{x} < 0` or `{x} <= 0`? (since if the value _was_ 0, we'd still presumably have a divide by zero error).