Unexpected tokens after conditionals should be an error instead of a warning.

Change-Id: I49d6d36ecf9a5f0d3d333422b3e4970192617c05
Reviewed-on: https://swiftshader-review.googlesource.com/5180
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>