Error checking for pyObjToNim conversions

[LemonBoy]

A small example:

let py = pyBuiltinsModule()
echo py.chr(65).to(int)

The chr function returns a string but the to(int) tries a conversion to int and PyLong_AsLongLong returns -1 to signal this. Checking for PyErr_Occurred or using the <type>_Check procedures may help here.