Check if a password already was prompted

[za-arthur]

This fixes infinite loop in case of an empty password.
The issue #299.

There is another PR #303 which fixes the issue. But it adds another infinite loop and it isn't consistent with the behavior when a user inputs wrong password.

In case of a wrong password pg_repack throws an error like:

ERROR: could not connect to database: connection to server on socket "/tmp/.s.PGSQL.5432" failed: FATAL:  password authentication failed for user "repack_user"

and it doesn't go to another loop and doesn't ask for another password.

In case of an empty password with this PR pg_repack will throw an error like:

ERROR: could not connect to database: connection to server on socket "/tmp/.s.PGSQL.5432" failed: fe_sendauth: no password supplied

Raising an error on empty password is consistent with other tools like psql.

[andreasscherbaum]

Does this need a test to make sure it works in the future?

[za-arthur]

It would be a bit tricky to create a test because I'm not sure how to provide a password to pg_repack in tests. It expects a user input.
AFAIK using pipe like the following doesn't work:

echo "test" | pg_repack ...