fixed an old buglet that caused bdb to be unable to
continue in the botframe, after a breakpoint was set.
the key idea is not to set botframe to the bottom level frame,
but its f_back, which actually might be None.
Additional changes: migrated old exception trick to use
sys._getframe(), which exists both in 2.1 and 2.2 .

Note: I believe Mark Hammond needs to look over his code now.
F5 correctly starts up in the debugger, but later on doesn't stop at a given
breakpoint any longer.

kind regards - chris