Spec doesn't consider error cases for `/_matrix/identity/v2/account/register`

Nobody has claimed this yet.

**Link to problem area**: https://matrix.org/docs/spec/identity_service/r0.3.0#post-matrix-identity-v2-account-register

Cropped up in
(Context: matrix-org/sydent#452, matrix-org/sydent#456).

**Issue**
To facilitate this request, the identity server must contact a remote homeserver and present an access token. This can fail in a number of ways (not exhaustive):

- We might be unable to resolve the homeserver's hostname.
- Our connection to the homeserver may fail or timeout.
- The homeserver may deny our request, if we were given an expired or invalid access token.
- The homeserver may respond with invalid JSON, or not include the information we require.

The spec does not consider any of these and only describes a 200 OK happy path.

I would like to return a meaningful http status code in these cases, but am not sure what The Done Thing is.

Read the linked Matrix identity-service specification section for POST /_matrix/identity/v2/account/register, then review the related Sydent issues matrix-org/sydent#452 and #456. Document how hostname, connection, authorization, and response-content failures should be represented, with the specification's completed error behavior agreed and recorded.

Written by the indexing model from the issue text.