Activity worker refactor

[dandavison]

This is pure refactoring for readability. I found it very helpful while creating a Nexus worker that is similar in some respects to the activity worker. All this PR does is move us to this pattern:

        try:
            await self._execute_activity(start, running_activity, completion) # 170 lines
        except BaseException as err:
            # 80 lines of error handling plus sending completion

i.e. separating the execution logic from the error handling, since it's very hard to read a try...except with 170 lines in the middle.

[cretz]

Not the biggest fan of de-inlining single-use logic for running an activity from run_activity when it is only used in one place. If anything, I could understand exception handling being de-inlined if it was really too long for you. But not enough disagreement to prevent the change.