Polished exception handling when importing extensions: - Warning about failed import is printed on one line. - For this warning the [extensions] section key is used, because the value doesn't have to be set. - Whan a --traceback is requested, exit immediately to not mix up problems when importing an extension with other problems.

#header#
# HG changeset patch
# User #author#
# Date #date|date#
# Node ID #node#
#parent%changesetparent#
#desc#

#diff#