feat: warning icon for form errors on tabs

[CiaranSanders]

This feature just simply introduces a small styling change for fieldset and inline tabs, so when a form error occurs, every fieldset and inline tab containing a field with an error now indicates it has an error.

Currently, when a form error occurs and fieldset tabs are in use, it is not clear in which tab the error occured under:

Example here a field under the "Other Information" tab had an error:

An example when multiple tabs have fields with form errors:

Example when multiple inlines with tabs had errors:

[CiaranSanders]

While testing this some more, I was getting weird behaviors in some model admins where the is_valid method was evaluating to False when it should not have been... Might be just a misunderstanding on my part on how that function is supposed to work.

But I replaced it's usage with total_error_count for determining if an inline tab has any errors under it.