Windows: use krb5_get_error_message instead of error_message

Windows: use krb5_get_error_message instead of error_message

krb5_get_error_message() can return more descriptive errors
based upon the context in which the error occurred by extracting
the cached error message from the krb5_context.  Since aklog
has a krb5_context, use that instead of error_message().

Use GetProcAddress to load krb5_get_error_message and
krb5_free_error_message function pointers because versions
of KFW prior to 3.2 do not export them.

LICENSE MIT

Change-Id: I12043ec1890bcf0f4d9566f35bb1228e579c49fe
Reviewed-on: http://gerrit.openafs.org/1520
Reviewed-by: Asanka Herath <asanka@secure-endpoints.com>
Tested-by: Asanka Herath <asanka@secure-endpoints.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>