Start a chat with oneself should work or fail more gracefully than 'you hit a bug'

[AmandineLP]

When trying to start a chat with oneself it either fails with the default 'Server overloaded or you hit a bug' popup (if the user has no ongoing 1:1 chats), or one first gets presented the list of existing 1:1s (I think) and then when selecting 'create new chat' get the same default error:

Creating a chat with oneself should either create a room with only oneself, or it should fail immediately and with a more explicit error

[lampholder]

p2 'cause in daily use this I can't imagine this being often encountered

The 'generic failure' message is generally terrible, though :(

[aaronraimist]

Related to #4062

[turt2live]

Also related: https://github.com/vector-im/riot-web/issues/4933

(maybe even a duplicate if we confirm https://github.com/vector-im/riot-web/issues/4933 as a feature)