raise exception when setting unknown breakpoint

The UndefinedReferenceException will be raised, if a breakpoint for an unkown
function is being set.

Conflicts:
	gdbpwny/GDB.py
enable setting of pending breakpoints by default

It can be disabled in the GDB constructor again. Upon setting a breakpoint for an unknown function will then result in an UndefinedReferenceException.

Conflicts:
	gdbpwny/GDB.py
reduce cyclomatic complexity for readability