Allow storage wrappers to through a forbidden exception with retry information for clients

[nickvergessen]

Current response on webdav requests:

<d:error xmlns:d="DAV:" xmlns:s="http://sabredav.org/ns" xmlns:o="http://owncloud.org/ns">
  <s:exception>OCA\DAV\Connector\Sabre\Exception\Forbidden</s:exception>
  <s:message>Access to this resource has been forbidden by law.</s:message>
  <o:retry xmlns:o="o:">true</o:retry>
  <o:reason xmlns:o="o:">Access to this resource has been forbidden by law.</o:reason>
</d:error>

@DeepDiver1975 @icewind1991
Required for FW storage wrapper.

Currently reason === exception message, not sure if we want to keep it like that, but for now that should be fine so we can continue the actual work.

[PVince81]

Make this DAVForbiddenException everywhere to avoid confusion ?

[nickvergessen]

Not really sure it helps a lot. DAV could still be the app or sabre...

[PVince81]

Code looks good otherwise 👍

[icewind1991]

Sounds like the perfect use case for HTTP 451: Unavailable For Legal Reasons 😄

[DeepDiver1975]

No need to overload - base already implements this as 403

[nickvergessen]

Right, I didn't extend in the beginning, so this was just a left over. will fix in a minute

[icewind1991]

👍 looks good

[DeepDiver1975]

I'm missing tests 👿

[nickvergessen]

@DeepDiver1975 added some tests to the dav stuff to test exception convertion and exception serialization