More precise definition of error handling

[phst]

Some errors (e.g. passing a null pointer, calling a function from the wrong thread) are currently undefined behavior (asserted only if checking is enabled), others (e.g. passing an object of a wrong type, wrong number of arguments, numeric overflow) raise Emacs signals. We should clearly state what the behavior is in each case and what principles it follows. (The current implementation is a bit unprincipled, but my general approach was to make errors defined that could also occur in Lisp code.)

[aaptel]

Specifying what is undefined, what signals and what simply returns is a good idea. I approve.