Rails config for raise on missing translations

Add a config to setup whether raise exception for missing translation or
not.
Merge pull request #13832 from kassio/setup-for-raise-missing-translations

Rails config for raise on missing translations
use the new clear_validators! api everywhere to reset validators in tests
Merge pull request #13855 from kuldeepaggarwal/reset_validators

use the new clear_validators! api everywhere to reset validators in tests
Replace File.exists? with File.exist?

To quell warnings on ruby 2.1
Merge pull request #13857 from teoljungberg/replace-file-exists-eh

Replace File.exists? with File.exist?