If mongoc_client_set_appname or mongoc_client_pool_set_appname is called more than once, or with a string that's too long, they currently return false and do nothing else. The mongoc_client_set_apm_callbacks function, which has similar behavior (it can also only be called once), logs an error when it fails. The set_appname functions should follow the same convention.

Be sure to update the docs as well.

Also update the tests which test these cases to call capture_logs