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
I used 'validates_uniqueness_of :name, :scope => :parent_id' and 'removed validate :uniqueness_of_fqname'
because it's less lines of code achieving the same + the message "fqname must be unique" was not exactly
helpful (what's a fqname? a domain? a namespace?).