Re: Nicer error when connecting to standby with hot_standby=off

The following review has been posted through the commitfest application:
make installcheck-world: not tested
Implements feature: tested, passed
Spec compliant: not tested
Documentation: not tested

I applied the patch to the latest master branch and run a test below. The error messages have been separated. Below is the test steps.

### setup primary server
initdb -D /tmp/primary/data
mkdir /tmp/archive_dir
echo "archive_mode='on'" >> /tmp/primary/data/postgresql.conf
echo "archive_command='cp %p /tmp/archive_dir/%f'" >> /tmp/primary/data/postgresql.conf
pg_ctl -D /tmp/primary/data -l /tmp/primary-logs start

### setup host standby server
pg_basebackup -p 5432 -w -R -D /tmp/hotstandby
echo "primary_conninfo='host=127.0.0.1 port=5432 user=pgdev'" >> /tmp/hotstandby/postgresql.conf
echo "restore_command='cp /tmp/archive_dir/%f %p'" >> /tmp/hotstandby/postgresql.conf
echo "hot_standby = off" >> /tmp/hotstandby/postgresql.conf
pg_ctl -D /tmp/hotstandby -l /tmp/hotstandby-logs -o "-p 5433" start

### keep trying to connect to hot standby server in order to get the error messages in different stages.
while true; do echo "`date`"; psql postgres -p 5433 -c "SELECT txid_current_snapshot();" sleep 0.2; done

### before the patch
psql: error: could not connect to server: FATAL: the database system is starting up
...

### after the patch, got different messages, one message indicates hot_standby is off
psql: error: could not connect to server: FATAL: the database system is starting up
...
psql: error: could not connect to server: FATAL: the database system is up, but hot_standby is off
...