Allow for error messages already being on the page due to server side rendering.

Check for existence of error message element.  If not present, create it.
Call addClass on parent div in all cases.  Ensures class is set regardless of pre-existing state.  addClass takes care of not adding the same class multiple times.

Added test.

#11
Moved logic into its own test
Handle exisitng errors from server-side
Version bump
Merge branch '2-0-stable' - We don't need 1.5 support
update gemspec to simple_form >= 2.0.3