Add ability to distinguish between non-existent aliases and other failures when calling `resolveRoom`

[squahtx]

When calling resolveRoom or lookupRoomAlias, there is currently no clean way to distinguish between a non-existent room alias and other failures, which ought to be handled differently.

This makes it difficult to correctly test whether a room already exists at a given alias. A use case for this might be a bot that is trying to create a room if it doesn't already exist.