Fixes __format__ check for existing formats
Fixes __format__ check for existing formats

Thanks to @mdbooms suggestion, I realized I fixed a single bug, but
added some poor code for others. This relies on error catching in
__format__ to determine if a specified format exists or not, instead
of a new `has_format` method

DougBurke/astropy,stargaser/astropy,joergdietrich/astropy,StuartLittlefair/astropy,bsipocz/astropy,bsipocz/astropy,StuartLittlefair/astropy,bsipocz/astropy,lpsinger/astropy,tbabej/astropy,astropy/astropy,kelle/astropy,aleksandr-bakanov/astropy,saimn/astropy,larrybradley/astropy,MSeifert04/astropy,saimn/astropy,tbabej/astropy,mhvk/astropy,astropy/astropy,stargaser/astropy,StuartLittlefair/astropy,AustereCuriosity/astropy,kelle/astropy,aleksandr-bakanov/astropy,joergdietrich/astropy,bsipocz/astropy,astropy/astropy,StuartLittlefair/astropy,kelle/astropy,larrybradley/astropy,aleksandr-bakanov/astropy,pllim/astropy,astropy/astropy,lpsinger/astropy,dhomeier/astropy,AustereCuriosity/astropy,MSeifert04/astropy,DougBurke/astropy,funbaker/astropy,MSeifert04/astropy,mhvk/astropy,MSeifert04/astropy,pllim/astropy,funbaker/astropy,joergdietrich/astropy,StuartLittlefair/astropy,lpsinger/astropy,AustereCuriosity/astropy,AustereCuriosity/astropy,stargaser/astropy,pllim/astropy,lpsinger/astropy,pllim/astropy,joergdietrich/astropy,dhomeier/astropy,joergdietrich/astropy,DougBurke/astropy,astropy/astropy,tbabej/astropy,funbaker/astropy,larrybradley/astropy,tbabej/astropy,dhomeier/astropy,mhvk/astropy,AustereCuriosity/astropy,DougBurke/astropy,mhvk/astropy,mhvk/astropy,tbabej/astropy,larrybradley/astropy,dhomeier/astropy,aleksandr-bakanov/astropy,pllim/astropy,kelle/astropy,kelle/astropy,larrybradley/astropy,saimn/astropy,lpsinger/astropy,stargaser/astropy,saimn/astropy,dhomeier/astropy,funbaker/astropy,saimn/astropy