changed error messages for compound mismatched inputs and outputs, and unrecognized operator
changed error messages for compound mismatched inputs and outputs, and unrecognized operator

StuartLittlefair/astropy,MSeifert04/astropy,saimn/astropy,lpsinger/astropy,aleksandr-bakanov/astropy,larrybradley/astropy,MSeifert04/astropy,larrybradley/astropy,larrybradley/astropy,saimn/astropy,lpsinger/astropy,aleksandr-bakanov/astropy,StuartLittlefair/astropy,bsipocz/astropy,pllim/astropy,aleksandr-bakanov/astropy,astropy/astropy,dhomeier/astropy,StuartLittlefair/astropy,bsipocz/astropy,astropy/astropy,larrybradley/astropy,mhvk/astropy,stargaser/astropy,mhvk/astropy,pllim/astropy,bsipocz/astropy,dhomeier/astropy,MSeifert04/astropy,stargaser/astropy,saimn/astropy,saimn/astropy,larrybradley/astropy,lpsinger/astropy,aleksandr-bakanov/astropy,lpsinger/astropy,astropy/astropy,mhvk/astropy,saimn/astropy,pllim/astropy,mhvk/astropy,dhomeier/astropy,astropy/astropy,pllim/astropy,pllim/astropy,mhvk/astropy,StuartLittlefair/astropy,dhomeier/astropy,bsipocz/astropy,stargaser/astropy,stargaser/astropy,lpsinger/astropy,StuartLittlefair/astropy,astropy/astropy,MSeifert04/astropy,dhomeier/astropy