make iterators safe for error handling

[simonbyrne]

This makes some minor changes to the iterator API functions:

• the callbacks can now return any integer type, or a bool
• errors from the callback are trapped and safely propagated back to the main process (doing this at the moment could leave the library in an invalid state: see https://docs.julialang.org/en/v1/manual/calling-c-and-fortran-code/#Creating-C-Compatible-Julia-Function-Pointers and https://discourse.julialang.org/t/what-happens-when-you-throw-an-error-from-a-cfunction/71510?u=simonbyrne): this was partially done in Define AttributeDict #948 (comment), but I figured it would be better to do this in a new PR
• there was an opportunity for a small optimization in the error stack printing function

[musm]

Why can't the reference always be an integer and instead of the isnothing checks, the value jlerr < 0 is checked done? It seems like this would avoid having to leave this as an Any reference.

[simonbyrne]

jlerr is the error object that is caught in the helper function, not an integer.

[simonbyrne]

we only check it if an error is thrown, so the overhead should be minimal.

[musm]

I see, thanks for the clarification.

[simonbyrne]

Any thoughts?

[musm]

Thanks!