Provide more detail in postmaster log for password authentication failures.

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.

src/backend/libpq/crypt.c

@@ -50,7 +50,11 @@ md5_crypt_verify(const Port *port, const char *role, char *client_pass,
 	/* Get role info from pg_authid */
 	roleTup = SearchSysCache1(AUTHNAME, PointerGetDatum(role));
 	if (!HeapTupleIsValid(roleTup))
+	{
+		*logdetail = psprintf(_("Role \"%s\" does not exist."),
+							  role);
 		return STATUS_ERROR;	/* no such user */
+	}
 
 	datum = SysCacheGetAttr(AUTHNAME, roleTup,
 							Anum_pg_authid_rolpassword, &isnull);
@@ -71,13 +75,20 @@ md5_crypt_verify(const Port *port, const char *role, char *client_pass,
 	ReleaseSysCache(roleTup);
 
 	if (*shadow_pass == '\0')
+	{
+		*logdetail = psprintf(_("User \"%s\" has an empty password."),
+							  role);
 		return STATUS_ERROR;	/* empty password */
+	}
 
 	CHECK_FOR_INTERRUPTS();
 
 	/*
 	 * Compare with the encrypted or plain password depending on the
-	 * authentication method being used for this connection.
+	 * authentication method being used for this connection.  (We do not
+	 * bother setting logdetail for pg_md5_encrypt failure: the only possible
+	 * error is out-of-memory, which is unlikely, and if it did happen adding
+	 * a psprintf call would only make things worse.)
 	 */
 	switch (port->hba->auth_method)
 	{
@@ -154,6 +165,9 @@ md5_crypt_verify(const Port *port, const char *role, char *client_pass,
 		else
 			retval = STATUS_OK;
 	}
+	else
+		*logdetail = psprintf(_("Password does not match for user \"%s\"."),
+							  role);
 
 	if (port->hba->auth_method == uaMD5)
 		pfree(crypt_pwd);