Another place to work around broken clients.

Don't add the collection to the cache if we don't have a synckey,
but requested it. Fixes issues in PING requests when the client
sends a PING for a collection that has not yet been SYNC'd.
Still add the collection or it possibly won't be picked up for PING.
Proper place to validate this.

If the server doesn't know the folder requested by the client, the
client did something really wrong. Log it, remove the offending collection
from the collections object and move on.