Decide on and implement consistent argument verification convention

[spbnick]

Some of the functions use assertions, some return AUSHAPE_RC_INVALID_ARGS, some assert that and other return values.

Decide on the rules and implement them.

[spbnick]

So far it seems that it is best to return AUSHAPE_RC_INVALID_ARGS for outside-facing functions only, and do assertions in the rest.