All wic exceptions are now inherited from new base exception
class WicError. It makes them easy to maintain and catch.
Processing of exceptions is done this way:
Known wic exceptions cause wic to print error message to stdout.
Unknown exceptions are not catched anymore and produce standard
python traceback.
Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
