fix Processor.process exception handling

[bfis]

Fixes the exception handling behavior for the Processor.process call by correctly chaining exceptions.

Reintroduces the benefits of #360.
Reintroduces the behavior of #370, since this is working as intended in python's concurrent/futures.
Retrains the fix for #438.
Fixes #649.

[nsmith-]

So the key is to add a new exception to the chain rather than attempt to modify the last exception's message?
We're approaching java levels of traceback length sadly, but it is too hazardous to try to edit them I think.

[andrzejnovak]

Filtering the traceback for a duplicate substring over some length would be too fragile?

[bfis]

We're approaching java levels of traceback length sadly, [...].

Yes, surprisingly, there is no tooling/help from python's side with handling/catching exceptions that are down the chain - it seems to me the original PEP-3134 did not concern itself too much with the handling of exception chains it introduced.

Filtering the traceback for a duplicate substring over some length would be too fragile?

Messing around with exception in such a away is a recipe for disaster since they have no fixed format and can do just about anything (e.g. tracebacks can be formatted very differently etc.). The underlying "duplication"/formatting issue lies in the internals of python`s concurrent.futures.ProcessPoolExecutor, so should be remedied there.