slight improvement to error message
slight improvement to error message

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