Errors should be logged when marking a server address bad.

[choubacha]

Describe the bug
When a server address is marked bad a line is logged but it does not contain the error or reason for marking it bad. This is a bug because it makes it very difficult to troubleshoot why our connections are breaking in production. Upstream from pgcat the client appears to receive a "connection reset by peer" error but we are unable to track down the specific error.

To Reproduce
Steps to reproduce the behavior:

1. Do something that triggers an error in the connection
2. See the "marked bad" log line
3. Observe that there is no line for the actual error that occurred

Expected behavior
The logged line should include at least a mention of the error.