Backends will need some way to signal that a precondition error occurred
(and specifying which one) without causing the server to return a 500.
This commit adds an exported function to create a specific error for
this. The existing error handling routine is slightly adapted to handle
this error in such a way that it returns the desired result.
Usage would be something like:
```
return "", carddav.NewPreconditionError(carddav.PreconditionNoUIDConflict)
```
which triggers the following HTTP response:
```
HTTP/1.1 409 Conflict.
Content-Type: text/xml; charset=utf-8.
Date: Thu, 10 Mar 2022 10:28:56 GMT.
Content-Length: 141.
Connection: close.
<?xml version="1.0" encoding="UTF-8"?>
<error xmlns="DAV:"><no-uid-conflict
xmlns="urn:ietf:params:xml:ns:carddav"></no-uid-conflict></error>
```
This response gets correctly recognized by e.g. Evolution (though it's
handling is not great).
The added error type is generic enough to be used for other stuff also.
As it is not exported (internal package), new functions for creating
such errors would have to be added.
