Use the new format of setting tags in view extension (Debian bug #361897)

Never exit directly from commands.dispatch(), but pass return code to caller. Usually the caller is commands.run(). Some extensions still use sys.exit(), this is catched, too. Fixed wrong return statement in commands.recover() yielding a zero exit code.

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.