Python SDK: report a series of failed exports once #85

After #86, the TypeScript SDK reports a series of failed exports one time. The first failure goes to onError. Later failures stay quiet until an export reaches the ingest again.

The Python SDK reports each failed batch. _ReportingSpanExporter and _ReportingLogExporter in _client.py and the metric exporter in _metrics.py call report_error for each FAILURE result. report_error also writes each report to the telemetry_dev logger. During an ingest outage, each batch adds one more report to on_error and to the log.

The on_error row in sdks/python/README.md already says that the SDK can merge repeated failures into one report. The Python exporters of one client can share one failure state, as the TypeScript exporters do. A batch that sends no request must not end the series.