convert: check for failed svn import in debugsvnlog and abort cleanly

'hg debugsvnlog' failed with a crash when using the uninitialized transport in
get_log_child if the import of the svn libraries had failed.

'convert' should never get as far as launching 'hg debugsvnlog' if the svn
libraries are missing, but by launching a subprocess there is risk that the
environment is mangled so the second import fails.

It is in principle also possible to launch the command manually.

hgext/convert/subversion.py

@@ -120,6 +120,10 b' def debugsvnlog(ui, **opts):'
     """Fetch SVN log in a subprocess and channel them back to parent to
     avoid memory collection issues.
     """
+    if svn is None:
+        raise util.Abort(_('debugsvnlog could not load Subversion python '
+                           'bindings'))
+
     util.setbinary(sys.stdin)
     util.setbinary(sys.stdout)
     args = decodeargs(sys.stdin.read())