webdav: ignore path and perm errors in PROPFIND

[WingGao]

PROPFIND can walk through directories, retrieving information about
each file. Unfortunately, the filesystem may deny access to the WebDAV
server for various reasons, such as the file truly not being readable
(e.g. a broken symlink), or because the server does not have permission
to read the file. PROPFIND should ignore these.

The current behaviour of the WebDAV server when encountering such issues
is to immediately stop its walk, and output an http 500. This leads to
poor behaviour with the builtin golang server, since the walk has likely
already written out its status header as part of serving the previously
walked files' properties. The server closes the response, also emitting
an error log.

While the error log is noisy, the closed response is truly an issue: the
xml returned to the client is invalid, which means that the response is
useless. It is not unreasonable to expect that a directory shared using
WebDAV has files which cannot be read for the reasons given above. The
shared directory becomes useless with the current behavior.

Rather than making directories with unreadable files useless, skip over
anything that is bad. A more nuanced solution to this problem could
likely involve indicating that the requested properties have problems,
or buffering the response prior to returning the failure. However, this
change is simple and a move in the right direction.

Fixes golang/go#16195
Fixes golang/go#43782