Quoted error messages are parsed incorrectly

[fintelia]

This code for some reason generates an UnexpectedCharacter error instead of producing empty output:

#if 0
#error "abc"
#endif

[Kangz]

This is because the lexer only parses valid GLSL tokens and # (for the preprocessor). Looking at two open-source GLSL preprocessors:

• ANGLE (WebGL) accepts #error "abc" because " gets tagged as an "other" token here and later the handling of #error appends all the token text until the next newline.
• glslang has pretty decent string parsing that's used both for #error and #include

We should define how good a string parsing we want to do, then use it for #error.