#4745 - warn on failed formatter calls

since qtconsole html output is limited, pandas choses to provide an
html repr on ipnb only and raise an exception from _repr_html_ when in qtconsole to fallback
to a text repr. This used to work fine, but #4745 now results in a warning message constantly
being emitted about the exception being raised.

@minrk, how about supressing a NotImplementedError as a form of contract?