Prevent connection managers from initiating sessions over failed connections

Before introducing this change, failed connections weren't addressed by connection managers in time, allowing session initiation to proceed. This resulted in further errors, and left the system in an invalid state where even attempting to shut down the connection manager would fail.

To reproduce this behavior (and to compare with behavior after the fix), attempt to open a connection to an invalid host (e.g.: adbcj:h2://badhost:9092/mem:test;MVCC=TRUE)

The patch includes fixes for both H2 and MySQL connection managers.