Network Server agentError calls log only to console and are hard to diagnose

The Network Server code uses an assertion-check utility routine called agentError()
under certain circumstances. When these agentError() calls arise, for example as
in DERBY-1454, there is no server side logging of the error except to the console.

The user application that hit DERBY-1454 had not been capturing console output and there
was no clue in the derby.log, this made the problem hard to track down when they
suddenly hit this boundary deep within their stress tests.

See also DERBY-743 for another issue with Network Server's agentError routine.