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
