bb.exceptions: don't show a repr of 'self'

bb.exceptions: don't show a repr of 'self'

Rather than treating self like an ordinary argument, showing a repr of its
value in the function spec when formatting the traceback entry, now we show
the class name for the method as a part of the function name. Example:

Old: bar(self=<some repr of Fooclass>, f=5)
New: Fooclass.bar(f=5)

Signed-off-by: Chris Larson <chris_larson@mentor.com>