mesa: make error checking optional in _mesa_lookup_shader_include()

mesa: make error checking optional in _mesa_lookup_shader_include()

This will be usefull when implementing glIsNamedStringARB() which
doesn't do error checking, it just returns false for invalid
lookups instead.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Witold Baryluk <witold.baryluk@gmail.com>