shaderc_includer_response blurb doesn't say how to signal includer errors

[dekimir]

I may be missing something, but it's not clear to me how to construct a shaderc_includer_response when the path is incorrect or content-reading fails.

[dekimir]

It appears that the convention is to return "" for the full path (and error message for the content) when an error occurs. This should be mentioned in the blurb.