Show actual errors instead of generic ones

[aine-etke]

Currently, Synapse-Admin shows generic errors when an error is returned from Synapse API. But Synapse Admin API uses error format from the Matrix specification.

Please add a global error handler, that will attempt to parse the Matrix error from the response, and if failed, just return the whole response as an error message.

The format should be the following:

<errcode> (<http status code>): <error>, example: M_MISSING_PARAM (400): parameter is missing

Example: invalid credentials

1. Attempt to login with invalid login/password
2. See error Authentication failed, please retry
3. Actual error returned from API: {"errcode":"M_FORBIDDEN","error":"Invalid username or password"}

It should show M_FORBIDDEN (403): Invalid username or password