core: do not let one failing site take down the whole instance.

If one site has invalid db password, it was impossible to tell from
the error message which the offending site was, since the whole boot
sequence came to a abrupt halt, and no site was responding (which in
turn turned my attention in the wrong direction, trying to figure out
why the db was misconfigured).

This fix crashes the booting for the offending site only, with a error
message hinting at which site it is, by stating the full details of
the target database (including user, schema etc, but NOT the
password) so the other sites can continue booting as normal.

The supervisor keeps trying though, so there's a bit of flooding in
the logs.. before it settles down.