Avoid segfault when raising Python errors that didn't set an exception.

[dfalbel]

This fixes #1404 in the sense that we no longer segfault when calling import_builtins()$globals().

The segfault happens because the call to globals() fails but never sets an exceptionType in Python side, which is expected by the py_fetch_error().

The reason why this call fails is still unkown to me (no backtrace makes it harder to figure it out), but it it seems that, the Python interpreter used by reticulate has no default module imported, causing the globals dictionary to not exist.

I think we might be missing somehting like:

reticulate/src/python.cpp

Lines 2967 to 2968 in 9a712cb

	PyObject* main = PyImport_AddModule("__main__");
	PyObject* globals = PyModule_GetDict(main);

during the initialization of reticulate's interpreter.

Previous reticulate versions raise:

> reticulate::import_builtins()$globals()
Error: <unknown error>