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

[babolivier]

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):

synapse/synapse/handlers/identity.py

Lines 631 to 633 in a0f48ee

	except HttpResponseException as e:
	logger.warning("Error contacting msisdn account_threepid_delegate: %s", e)
	raise SynapseError(400, "Error contacting the identity server")

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:

synapse/synapse/handlers/identity.py

Lines 535 to 537 in a0f48ee

	except HttpResponseException as e:
	logger.info("Proxied requestToken failed: %r", e)
	raise e.to_synapse_error()