submit: continue superseding on per-SR HTTP 403 errors

[SAY-5]

Catch HTTP 403 on each individual change_request_state call when superseding existing SRs, log a warning, and continue with the remaining requests instead of aborting the whole loop. Fixes #2130.

[dmach]

Wouldn't be better to extend change_request_state() with an argument, say can_fail: bool = False instead of adding a new function?

[SAY-5]

Good point, happy to refactor as a can_fail: bool = False kwarg on change_request_state(). I'll push that shortly.