Support alternate sign in error message when email record does not exist

By default, the nonexistent error is still identical to the :invalid
message, and must be customized by the developer to implement.

config/locales/en.yml

@@ -18,6 +18,7 @@ en:
       unauthenticated: 'You need to sign in or sign up before continuing.'
       unconfirmed: 'You have to confirm your account before continuing.'
       locked: 'Your account is locked.'
+      nonexitent: 'Invalid email or password.'
       invalid: 'Invalid email or password.'
       invalid_token: 'Invalid authentication token.'
       timeout: 'Your session expired, please sign in again to continue.'

lib/devise/strategies/database_authenticatable.rb

@@ -6,7 +6,7 @@ module Devise
     class DatabaseAuthenticatable < Authenticatable
       def authenticate!
         resource = valid_password? && mapping.to.find_for_database_authentication(authentication_hash)
-        return fail(:invalid) unless resource
+        return fail(:nonexitent) unless resource
 
         if validate(resource){ resource.valid_password?(password) }
           resource.after_database_authentication