If required to reconnect, reuse previous IAM password if possible

[robins]

If required to reconnect, reuse previous IAM password if possible (for e.g. not yet expired / if prev password was a pan-database user etc),

Currently it tries to blindly fetch a new IAM password, which is avoidable.

[robins]

As of 8aad87d we're reusing IAM authentication, however, when the token expires, currently we aren't checking for failure (and re-requesting a new token).

t2=# \c t
FATAL:  IAM Authentication token has expired
FATAL:  password authentication failed for user "IAMA:redshift2"
Previous connection kept
t2=#