bitbake: fix wrong usage of format_exc

bitbake: fix wrong usage of format_exc

First parameter of traceback.format_exc is a 'limit' - a number
of stracktraces to format.

Passing exception object to format_exc is incorrect, but it works in
Python 2 as this code from traceback module works:
    while tb is not None and (limit is None or n < limit):
Comparing integer counter n with the exception object in Python 2
always results in True. However, in Python 3 it throws exception:
    TypeError: unorderable types: int() < <Exception type>()

As format_exc is used in except block of handling another
exception this can cause hard to find and debug bugs.

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>