I actually don't see what the big deal is with ERR_PTR and friends.
In higher level languages like OCaml, Java, etc., when you encounter an unrecoverable error in a function, you throw an exception. Then the function has no return value-- control just passes directly to the relevant catch() block.
ERR_PTR is the same thing. Normally, the function would return a foo pointer, but an unrecoverable error happened. So you get an error code instead. As a bonus, if you forget to check for the error code, you get a guaranteed crash (well, if some bonehead hasn't allowed the page starting at address 0 to be mapped). I say "bonus" because the alternative is usually a nondeterministic crash.