Make 'type lookup' print an error message instead of complete radio silence...

Make 'type lookup' print an error message instead of complete radio silence when it can't find a type matching user input

It would be fun to make it provide suggestions (e.g. 'can't find NString, did you mean NSString instead?'), but this worries me a little bit on the account of just how thorough of a type system scan it would have to do

llvm-svn: 264343