[PATCH] improve the pg_upgrade error message

While looking into one of the pg_upgrade issue, I found it

challenging to find out the database that has the datallowconn set to

'false' that was throwing following error:

*"All non-template0 databases must allow connections, i.e. their
pg_database.datallowconn must be true"*

edb=# create database mydb;

CREATE DATABASE

edb=# update pg_database set datallowconn='false' where datname like 'mydb';

UPDATE 1

Now, when I try to upgrade the server, without the patch we get above

error, which leaves no clue behind about which database has datallowconn

set to 'false'. It can be argued that we can query the pg_database

catalog and find that out easily, but at times it is challenging to get

that from the customer environment. But, anyways I feel we have scope to

improve the error message here per the attached patch.

With attached patch, now I get following error:

*"All non-template0 databases must allow connections, i.e. their
pg_database.datallowconn must be true; database "mydb" has datallowconn set
to false."*

Regards,

Jeevan Ladhe