Error when trying to join an empty room should be more explicit (SYN-678)

[matrixbot]

This issue has been migrated from #1533.

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Submitted by @​matthew:matrix.org
It is currently {"errcode":"M_UNKNOWN","error":"No known servers"} - i am having to string match on the error message to display a more coherent one in Vector :(

(Imported from https://matrix.org/jira/browse/SYN-678)