Fix failure path for invalid backer values
Controller spec for invalid backer values

post is called twice. The second time is after the :errors stub is
in place. Ideally this would be called once.
Missing l10n for new backer error

Taken from 7c9d064

An more specific error message could be set in the controller, for
example:
flash[:failure] = @backer.errors.full_messages.first
Merge pull request #170 from ianfleeton/invalid-backer-values

Invalid backer values