Re-use, rather than duplicate, error exit code.

This it less important than in Py3 as most optimisable cases are already covered by PyInt, but long objects might still pop up arbitrarily in Py2, so it might be worth it.

Closes #1438.

directly call into CPython's Unicode writers to format float/int values, rather than going all the way through PyObject_Format() and its method lookups

fix docs example: getArea instead of undefined getLength

Replace "int(val) == 0" by "not val" for gdb.Value instances