Removed fatal errors from Py_Initmodule4() (and thus from

Removed fatal errors from Py_Initmodule4() (and thus from
Py_Initmodule(), which is a macro wrapper around it).

The return value is now a NULL pointer if the initialization failed.
This may make old modules fail with a SEGFAULT, since they don't
expect this kind of failure.  That's OK, since (a) it "never" happens,
and (b) they would fail with a fatal error otherwise, anyway.

Tons of extension modules should now check the return value of
Py_Initmodule*() -- that's on my TODO list.