debugger() call results in additional exception from Bdb

When inserting a debugger() call into a user program, a Bdb exception occurs if the debugger is exited while the program still running.  This is very likely because the debugger() call is implemented using the pdb module rather than by subclassing Bdb (or possibly subclassing Pdb).  In any case, the additional exception is confusing and needs to be eliminated.