serf error handling for locks without authn

http://subversion.tigris.org/issues/show_bug.cgi?id=4368

Locking with anonymous http fails because there is no username. At
present mod_dav_svn returns "401 Unathorized" however

http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.2

says that a 401 response MUST include a WWW-Authenticate header and
apache's response doesn't include such as header as authn is not
configured. That looks as if mod_dav_svn is returning invalid
HTTP. Should we return something else? Perhaps "501 Not Implemented"?

The WebDAV RFC 2518 gives "412 Precondition Failed" as a possible error,
although I'm not clear whether that would be suitable for this request.
If I change mod_dav_svn to return a 412 instead of 401 I see what looks
like a ra_serf bug:

$ svn lock http://localhost:8888/obj/repo/A/f
'f' locked by user '(null)'.

The 412 is treated as success! The code in
libsvn_ra_serf/locks.c:determine_error looks dodgy:

213 static svn_error_t *
214 determine_error(svn_ra_serf__handler_t *handler,
215 svn_error_t *err)
216 {
217 {
218 apr_status_t errcode;
219
220 if (handler->sline.code == 423)
221 errcode = SVN_ERR_FS_PATH_ALREADY_LOCKED;
222 else if (handler->sline.code == 403)
223 errcode = SVN_ERR_RA_DAV_FORBIDDEN;
224 else
225 return err;
226
227 /* Client-side or server-side error already. Return it. */
228 if (err != NULL)
229 return err;
230
231 /* The server did not send us a detailed human-readable error.
232 Provide a generic error. */
233 err = svn_error_createf(errcode, NULL,
234 _("Lock request failed: %d %s"),
235 handler->sline.code,
236 handler->sline.reason);
237 }
238
239 return err;
240 }

Lines 224-225 mean that all the following code is never executed and
anything other than 423 or 403 is going to get lost. I can't simply
remove those two lines because errcode needs a value for
svn_error_createf.

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