Use same string on failed login regardless of whether account exists when in paranoid mode

[JasonBarnabe]

Previously reported in #4568. devise is using devise.failure.invalid when you attempt to log in with an existing identifier but a wrong password, while it uses devise.failure.not_found_in_database when the identifier does not exist. In the default en.yml provided by devise, these strings are the same, but if you are using a different localization file, they may not be (see tigrish/devise-i18n#223).

If you have enabled paranoid mode, the same localization key should be used for both cases.

[tegon]

I haven't understood what the problem was on the previous issue, but now I do 😅 . Thanks, @JasonBarnabe.
This change seems reasonable to me, but I think we would have to do it in a major version since it isn't backward compatible.
Would you be willing to work on this?

[tegon]

Closed via #4854