Fix validation methods in relevant admin classes

This issue fixes #205 and similar issues the other affected admin classes. 

Unfortunately, when validation fails in the Sonata form, only a global error message is displayed at the top of the form.  The field-specific error message div remains puzzlingly empty.  I ran out of time trying to find a fix myself, but noticed that Sonata has this issue open:

https://github.com/sonata-project/SonataAdminBundle/issues/1076

...which, hopefully, will be addressed at some point in the future.