optionally print tracebacks when the Mercurial hook swallows an exception

If `ui.traceback=True` is set on the server, this prints the stacktrace for the
exception on the client side.  Otherwise, nothing happens.  I tried allowing the
exception to propagate back to Mercurial, but then the client sees this message
with 4.4.2 and 4.6.1:

    abort: remote error:
    Mercurial/Python process ends with return code 1

Something odd changed when upgrading from CentOS 7.4 to 7.5 around forwarding
requests from the loopback address that I don't fully understand.  First, we
were getting a ValueError from inside `opener.open()` saying that 'localhost'
didn't match the host listed in the SSL certificate.  That wasn't visible until
adding this.

Then what happened is a connection refused out of the same function, so the
traceback is added to the other handler too.  Running the equivalent command on
the command line from the 'vcs' host stopped working in 7.5:

  $ curl https://vcs.domain.com/hook/hg/?ping=true
  curl: (7) Failed connect to vcs.domain.com:443; Connection refused

But it works when run on another machine targeting that same 'vcs' host.  Adding
another firewall rule allows everything to work from the 'vcs' host again:

  $ iptables -t nat -I OUTPUT -p tcp -o lo --dport 443 -j REDIRECT --to-ports 8443

scm-plugins/scm-hg-plugin/src/main/resources/sonia/scm/python/scmhooks.py

@@ -78,8 +78,10 @@ def callHookUrl(ui, repo, hooktype, node):
       printMessages(ui, msg.splitlines(True))
     else:
       ui.warn( "ERROR: scm-hook failed with an unknown error\n" )
+    ui.traceback()
   except ValueError:
     ui.warn( "scm-hook failed with an exception\n" )
+    ui.traceback()
   return abort
 
 def callback(ui, repo, hooktype, node=None, source=None, pending=None, **kwargs):