tools: moveconfig: exit with error message for not clean directory

When the source tree is not clean, this tool raises an exception
with a message like follows:

Traceback (most recent call last):
  File "tools/moveconfig.py", line 939, in <module>
    main()
  File "tools/moveconfig.py", line 934, in main
    move_config(config_attrs, options)
  File "tools/moveconfig.py", line 808, in move_config
    while not slots.available():
  File "tools/moveconfig.py", line 733, in available
    if slot.poll():
  File "tools/moveconfig.py", line 645, in poll
    self.parser.update_dotconfig(self.defconfig)
  File "tools/moveconfig.py", line 503, in update_dotconfig
    with open(autoconf_path) as f:
IOError: [Errno 2] No such file or directory: '/tmp/tmpDtzCgl/include/autoconf.mk'

This does not explain what is wrong.  Show an appropriate error
message "source tree is not clean, please run 'make mrproper'"
in such a situation.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>