Check if emails are Bugzilla users when needinfo-ing them

[Dexterp37]

From #505 :

I've been playing with the BMO API over in https://bugzilla.mozilla.org/show_bug.cgi?id=1792701 and think the "proper" fix would be to, on failure, check the get user endpoint for users who can log in (can_login: true) and retry with only those users needinfo'd. And then file without any ni?s.