Adjust logging levels for LDAP failures

[tvernum]

This change increases the logging level for LDAP failures in 3 cases:

1. When a search fails, the failure details are now logged at DEBUG
   rather than TRACE. The errors are useful for diagnostic purposes and
   it should not be necessary to turn on TRACE to see them

2. When a failure occurs when attempting to bind using the configured
   "bind_dn" and password, this is now a WARN rather than DEBUG. Failures
   for user supplied credentials (e.g. basic auth) are still logged as
   DEBUG because these are typically not under the control of the cluster
   administrator.

3. When a failure occurs while attempting to retrieve a connection
   from an LDAP connection pool, this will be logged at a WARN level.
   This is almost always an error that the cluster administrator should
   be aware of and seek to resolve.

In some cases this may cause 2 sets of log messages (one from LDAP and
one from the authentication service) however it is not always the case
that both are logged, and even when they are they may have different
levels of detail (for example, the authentication service message does
not always include the DN of the bind user).

Backport of: #76058