Implement MSC3743 (Standardized error response for unknown endpoints)

[clokep]

MSC3743 describes some "standard error responses" that a homeserver is to implement. Synapse is partially compliant, but needs to:

1. Return a 404 instead of a 400 in some cases.
2. Return a 404 + JSON body for all /_matrix/ sub-resources (media and key specifically don't return properly now.)
3. Return a 405 + JSON body for unknown methods on known resources. (Currently an HTML body is returned.)

This is a bit annoying since Synapse has two separate ways to route to resources:

• Twisted Resource (which contain a map to a set of sub-resources).
• Custom routing via RestServlet.PATTERNS.

[clokep]

I think to have a reasonable implementation of this we should first turn everything internal in the Synapse code to a RestServlet (essentially, don't import Resource directly anywhere). Not sure how terrible that would be.

[clokep]

Most of this work got done in #14599. They're essentially duplicates.