Handle errors (e.g. couchdb server down)

[bkniffler]

Necessary to catch errors due to connection issue to couchdb instance. Errors can then be caught by something like

app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).send('Something broke!');
});

If you accept this PR, I'd be super-grateful about a new version publish :)

Thanks for the great work.

ps. It would've been even a greater experience if this repo had provided a couchdb config file, something (feel free to add it to readme!) like:

[chttpd]
bind_address = any
port = 5984
authentication_handlers = {couch_httpd_auth, proxy_authentication_handler}, {couch_httpd_auth, cookie_authentication_handler}, {couch_httpd_auth, default_authentication_handler}

[httpd]
bind_address = any

[couch_httpd_auth]
proxy_use_secret = true
secret = SOME_SECRET

Its easy to confuse the httpd and chttpd section of authentication_handlers.