[analyzer] Emit an error rather than assert on invalid checker option input

[analyzer] Emit an error rather than assert on invalid checker option input

Asserting on invalid input isn't very nice, hence the patch to emit an error
instead.

This is the first of many patches to overhaul the way we handle checker options.

Differential Revision: https://reviews.llvm.org/D57850

llvm-svn: 355704