Show error message when a domain / namespace already exists

Previously, webui would display a success message informing
the domain / namespace was successfuly created, although
it wasn't.

https://github.com/ManageIQ/manageiq/issues/267
coding style: add space after comma
Merge pull request #349 from mzazrivec/fix_duplicate_domains_namespaces

Show error message when a domain / namespace already exists