fix: finish the PGconn upon connection failure

fix: finish the PGconn upon connection failure

Attaching the non-finished PGconn to exceptions raised in connect() is
causing problem, as described in issue #565, because the PGconn might
not get finished soon enough in application code and the socket would
remain open.

On the other hand, just removing the pgconn attribute from Error would
be a breaking change and we'd loose the inspection features introduced
in commit 9220293dc023b757f2a57702c14b1460ff8f25b0.

As an alternative, we define a new PGconn implementation that is
error-specific: it captures all attributes of the original PGconn and
fails upon other operations (methods call). Some attributes have a
default value since they are not available in old PostgreSQL versions.

Finally, the (real) PGconn is finished before raising exception in
generators.connect().