Vague error when trying to create a room alias that already exists

[anoadragon453]

Description

A generic error is shown when trying to add an alias to a room that already has the alias, rather than an error message that explicitly states that.

The response received from Synapse is a 409 - Conflict.

Steps to reproduce

• Create an alias in a room that alias already exists.

[anoadragon453]

Possibly related: matrix-org/matrix-react-sdk#835

[Biep]

This occurs if the alias is in use anywhere - not necessarily for the same room.
The message might link to the room that has the alias - maybe it is what people want, only they didn't know the room already existed, and therefore created another one.
(Linking doesn't break privacy, as the user could type in the address and try to go there anyway.)

[turt2live]

We replaced this with a tooltip saying it is taken