Log errors on UNIMPLEMENTED() rather than aborting.
For now ASSERT() will still abort.

Change-Id: I4db66934d0cd69b557fd2989e0c120a5fa372b99
Reviewed-on: https://swiftshader-review.googlesource.com/2691Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Greg Hartman <ghartman@google.com>