[CMake] Tweak r197697 to handle CMake finding the wrong llvm-config/tblgen.

- If llvm-config fails, output an error to the user rather than allowing
  errors to cascade.
- Always get llvm-tblgen from llvm-config's bindir.

Turns out my PATH points to a really old version of LLVM; both of these
fell out of trying to make this experience nicer.