Improve exception error message for bad unit input
Improve exception error message for bad unit input

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