Revert some validation error string to previous state

Commit a33f99a changed one string that is actually used by
barclamp-test:
https://github.com/crowbar/barclamp-test/blob/673b2ed830bea7d6b565495859e7625466a87a22/bin/barclamp_test.rb#L110

The string was:
  [/attributes/foobar] key 'foobar:' is undefined.
instead of:
  key 'foobar:' is undefined.

We also need to add the trailing \n (which could arguably matter in the
UI).
Merge pull request #804 from vuntz/tweak-validation-string

Got +1 => merging