Improve UX for exported funcs that do not declare errors.

[TripleDogDare]

Presumably we know what errors a function is tainted with.

function "FnName" is exported, but does not declare any error codes

This error message could tell you which errors to decorate the function with or to add the none tag if there are none.

1. For functions with error codes, this removes the step of adding Errors: none and re-running the analyzer to find the actual missing error codes.
2. For functions with that always return nil errors, this informs the user of the existence of a none tag without digging through documentation.