Handle permission failures in Discord calls better

For example:

```
2022-01-18 01:11:55.315 WARN 1 --- [oundedElastic-2] m.i.m.integrations.RoleHandlingService : Failure during processInitialRoleAssignments

discord4j.rest.http.client.ClientException: PUT /guilds/[redacted]/members/[redacted]/roles/[redacted] returned 403 Forbidden with response {code=50013, message=Missing Permissions}
at discord4j.rest.http.client.ClientResponse.clientException(ClientResponse.java:171) ~[discord4j-rest-3.2.1.jar:3.2.1]
```

Also the 500 error template didn't have access to the expected `message`.

Start by locating RoleHandlingService and the 500 error template referenced by the report, then trace how the Discord4J 403 response reaches them. Confirm the completed behavior against the Missing Permissions example and ensure the 500 template can access the expected message.

Written by the indexing model from the issue text.