Instead of always warning on invalid pragmas, only warn when the pragma
type is not recognized and error when the syntax is invalid.

Fixes:
dEQP-GLES2.functional.shaders.preprocessor.pragmas.invalid_pragma_invalid_debug_vertex
dEQP-GLES2.functional.shaders.preprocessor.pragmas.invalid_pragma_invalid_debug_fragment
dEQP-GLES2.functional.shaders.preprocessor.pragmas.invalid_pragma_invalid_token_vertex
dEQP-GLES2.functional.shaders.preprocessor.pragmas.invalid_pragma_invalid_token_fragment

BUG=angleproject:989

Change-Id: Ibd584dc08a2436e163dfc52eeffdf2dac8a22cb8
Reviewed-on: https://chromium-review.googlesource.com/267639Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>