handle statistics update even if statistics dict not initialized

[2timesjay]

Issue: #507

Handle KeyError that occurs in edge cases by changing instances of self.statistics[key] += ... to self.statistics[key] = self.statistics.get(key, ...) + ...

This will eliminate keyerrors.

In the cases where statistics isn't initialized with begin(), this will still not ensure that it's correct, but will ensure that we retry instead of crashing.

Alternately,

            self.statistics["idle_for"] = rs.idle_for
            self.statistics["attempt_number"] = rs.attempt_number

might be more accurate.

[mergify]

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