Improve error handling in svn_ra_serf__replay_range()

Hi,

I was looking at the issue #4100: "svnrdump dump -rN:M
<URL-deleted-in-HEAD>' fails" [1].

<issue-snip>
[[[
$
URL=http://svn.apache.org/repos/asf/subversion/trunk/contrib/server-side/mod_setlocale

$ svnrdump dump $URL -r1232154 > /dev/null
* Dumped revision 1232154.

$ svnrdump dump $URL -r1232155 > /dev/null
* Dumped revision 1232155.

$ svnrdump dump $URL -r1232154:1232155 > /dev/null
* Dumped revision 1232154.
subversion/svnrdump/svnrdump.c:433: (apr_err=160013)
subversion/libsvn_ra/ra_loader.c:1184: (apr_err=160013)
subversion/libsvn_ra_neon/util.c:1323: (apr_err=160013)
subversion/libsvn_ra_neon/util.c:596: (apr_err=160013)
svnrdump: E160013:
'http://svn.apache.org/repos/asf/subversion/trunk/contrib/server-side/mod_setlocale'
path not found
]]]
</issue-snip>

I could see the error "path not found" with ra_neon but ra_serf doesn't
return any error whereas it should.

This patch will fix this issue by handling the errors while processing
connections defined in serf context.

Attached the patch and log message.

[1] http://subversion.tigris.org/issues/show_bug.cgi?id=4100

Thanks & Regards,
Vijayaguru