Send/receive error details with widgets (#4492)
* Send/receive error details with widgets
* Fix embedded client tests
* Use all properties of error responses
* Lint
* Rewrite ternary expression as if statement
* Put typehints on overridden functions
* Lint
* Update matrix-widget-api
* Don't @link across packages
as gendoc fails when doing so.
* Add a missing docstring
* Set widget response error string to correct value
* Test conversion to/from widget error payloads
* Test processing errors thrown by widget transport
* Lint
* Test processing errors from transport.sendComplete
