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.

removing b
this update spans a branch affecting the following files:
 b
aborting update spanning branches!
(use 'hg merge' to merge across branches or 'hg update -C' to lose changes)