Everywhere that was actually using the validator had exactly the same pattern:
name = inputname;
throw IllegalArgumentException("exactly the same error message");
so it seemed to make sense to me to pull that into the method. The boolean method is still in SolrIdentifierValidator, although it's now private, but I can make it public again if that helps things.
re changing the Exception type, I agree that it's not ideal, but keeping it as IAE ended up losing information when exceptions were propagated in TestCollectionAPI. Looking at it again, though, this is mainly due to the tests there not using CollectionsAdminRequests explicitly, so there's no client-side checking. I'll see if I can change it back again.