`Illegal offset type` exception should mention the type

[mvorisek]

Description

https://3v4l.org/m069E

[marcosmarcolin]

I agree with you! @cmb69 Do you think this is viable?

If yes, I would like to try to apply this change.

[cmb69]

If yes, I would like to try to apply this change.

Yeah, sure, give it a try. It is not my decision, though, whether a respective PR will be accepted (albeit it is not unlikely; looks like a useful feature).