http/500 when database is not responding

[al4]

Exceptions such as this should still return 500, but give a nicer error message

[2016-11-17 11:21:46 +0000] [8297] [ERROR] Error handling request /info/packages/versions
Traceback (most recent call last):
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/gunicorn/workers/sync.py", line 135, in handle
    self.handle_request(listener, req, client, addr)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/gunicorn/workers/sync.py", line 176, in handle_request
    respiter = self.wsgi(environ, resp.start_response)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/flask/app.py", line 2000, in __call__
    return self.wsgi_app(environ, start_response)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/flask/app.py", line 1991, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/flask/app.py", line 1567, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/flask/app.py", line 1988, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/flask/app.py", line 1641, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/flask/app.py", line 1544, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/flask/app.py", line 1639, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/flask/app.py", line 1625, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/orlo/route_info.py", line 123, in info_package_versions
    result = q.all()
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2613, in all
    return list(self)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2761, in __iter__
    return self._execute_and_instances(context)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2776, in _execute_and_instances
    result = conn.execute(querycontext.statement, self._params)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 914, in execute
    return meth(self, multiparams, params)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/sql/elements.py", line 323, in _execute_on_connection
    return connection._execute_clauseelement(self, multiparams, params)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1010, in _execute_clauseelement
    compiled_sql, distilled_params
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1146, in _execute_context
    context)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1341, in _handle_dbapi_exception
    exc_info
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/util/compat.py", line 202, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb, cause=cause)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1139, in _execute_context
    context)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/engine/default.py", line 450, in do_execute
    cursor.execute(statement, parameters)
OperationalError: (psycopg2.OperationalError) server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.
 [SQL: 'SELECT package.name AS package_name, package.version AS package_version \nFROM package JOIN (SELECT package.name AS name, max(package.stime) AS max_stime \nFROM package \nWHERE package.status = %(status_1)s GROUP BY package.name) AS anon_1 ON anon_1.max_stime = package.stime AND anon_1.name = package.name GROUP BY package.name, package.version'] [parameters: {'status_1': 'SUCCESSFUL'}]