Synapse doesn't pass errors from the identity server through to clients when verifying a phone number

This issue has been migrated from [#11016](https://github.com/matrix-org/synapse/issues/11016).

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If an identity server responds to a request with an error, e.g. with a message saying a validation token is invalid etc, Synapse doesn't pass it through to clients. Instead, it responds with a generic "Error contacting the identity server" error (with the `M_UNKNOWN` code):

https://github.com/matrix-org/synapse/blob/a0f48ee89d88fd7b6da8023dbba607a69073152e/synapse/handlers/identity.py#L631-L633

This creates a confusing user experience because clients can't do much other than printing out that generic message, which doesn't make it clear where the issue comes from:


Note that we correctly pass the error through when requesting a token for a phone number, so I don't think there should be an issue with doing the same here:

https://github.com/matrix-org/synapse/blob/a0f48ee89d88fd7b6da8023dbba607a69073152e/synapse/handlers/identity.py#L535-L537