Handling the bad/lost connection of a database server.

Made backend changes for:
* Taking care of the connection status in the psycopg2 driver. And, when
  the connection is lost, it throws a exception with 503 http status
  message, and connection lost information in it.
* Allowing the flask application to propagate the exceptions even in the
  release mode.
* Utilising the existing password (while reconnection, if not
  disconnected explicitly).
* Introduced a new ajax response message 'service_unavailable' (http
  status code: 503), which suggests temporary service unavailable.

Client (front-end) changes:
* To handle the connection lost of a database server for different
  operations by generating proper events, and handle them properly.

Removed the connection status check code from different nodes, so that
- it generates the proper exception, when accessing the non-alive
  connection.

Fixes #1387

web/pgadmin/browser/server_groups/servers/databases/event_triggers/__init__.py

@@ -16,9 +16,8 @@ from flask import render_template, make_response, request, jsonify
 from flask_babel import gettext
 from pgadmin.browser.collection import CollectionNodeModule
 from pgadmin.browser.utils import PGChildNodeView
-from pgadmin.utils.ajax import make_json_response, \
-    make_response as ajax_response, internal_server_error
-from pgadmin.utils.ajax import precondition_required
+from pgadmin.utils.ajax import make_json_response, internal_server_error, \
+    make_response as ajax_response
 from pgadmin.utils.driver import get_driver
 
 from config import PG_DEFAULT_DRIVER
@@ -188,22 +187,12 @@ class EventTriggerView(PGChildNodeView):
 
         @wraps(f)
         def wrap(*args, **kwargs):
-            # Here args[0] will hold self & kwargs will hold gid,sid,did
+
+            # Here - args[0] will always hold self & kwargs will hold gid, sid, did
             self = args[0]
             self.manager = get_driver(PG_DEFAULT_DRIVER).connection_manager(kwargs['sid'])
             self.conn = self.manager.connection(did=kwargs['did'])
-
-            # If DB not connected then return error to browser
-            if not self.conn.connected():
-                return precondition_required(
-                    gettext(
-                        "Connection to the server has been lost!"
-                    )
-                )
-
-            ver = self.manager.version
-            if ver >= 90300:
-                self.template_path = 'event_triggers/sql/9.3_plus'
+            self.template_path = 'event_triggers/sql/9.3_plus'
 
             return f(*args, **kwargs)