We tell people to examine the postmaster log if they're unsure why they are
getting auth failures, but actually only a few relatively-uncommon failure
cases were given their own log detail messages in commit 64e43c59.
Expand on that so that every failure case detected within md5_crypt_verify
gets a specific log detail message.  This should cover pretty much every
ordinary password auth failure cause.

So far I've not noticed user demand for a similar level of auth detail
for the other auth methods, but sooner or later somebody might want to
work on them.  This is not that patch, though.