io.registry: added custom error class
io.registry: added custom error class

fixes #4831 by replacing use of `Exception` with a new custom `IORegistryError` inherited from `ValueError`.

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