The oldContent field of message objects is unreliable

[object-Object]

Since Discordia weakly caches messages, message.oldContent is unreliable. If the message is uncached, the following code, when run in the messageUpdate event, will fail on line 2, not line 1:

assert(message.oldContent)
assert(message.oldContent[message.editedTimestamp])

This leaves two options:

1. If a message is uncached, just tell the user that the old content couldn't be found.

2. Manually cache all messages in the database.