fix some logging glitches in the WinNT MPM:

the MPM's equivalent of "reached MaxClients" is now ERR instead of
WARNING

the message when the MPM child exits due to excessive errors is
raised from ERR to CRIT

the caller of mpm_get_completion_context() could log random error
values (GetLastError() or rv), so solve as follows:

. mpm_get_completion_context() always logs its errors
. caller only logs when it is taking a rare action on such errors, and
  doesn't pretend to know the error code of the last such error


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1090605 13f79535-47bb-0310-9956-ffa450edef68