catch edge cases

[simonLeary42]

#172

once #233 is merged, we can reasonably assume no one will be submitting forms multiple times, so it's easier to justify throwing errors instead of silently failing.

situation	behavior before	behavior after	covered by tests ?
new duplicate group creation request	allowed	error	no
new group creation request but group already exists	noop	error	no
new group creation request but owner requested account deletion	noop	error	no
approve nonexistent group creation request	allowed	error	no
approve group creation request but group already exists	noop	error	no
approve group creation request but owner requested account deletion	noop	error	no
deny group creation request but group already exists	noop	error	no
deny nonexistent group creation request	noop	log and noop	no
cancel nonexistent group join request	noop	log and noop	no
cancel nonexistent group join request	error	error	no
deny nonexistent group join request	noop	error	no
remove absent user from group	noop	log and noop	no
remove group owner from group	error	error	no
new group join request but user already in group	log and noop	error	no
new duplicate group join request	log and noop	error	no
new group join request but user requested account deletion	error	error	no