execute_batch, start_and_wait: generic "retry later" functionality

[soxofaan]

execute_batch/start_and_wait now ignores a small subset of errors that could occur:

openeo-python-client/openeo/rest/job.py

Lines 258 to 264 in 641ed51

	except requests.ConnectionError as e:
	soft_error("Connection error while polling job status: {e}".format(e=e))
	continue
	except OpenEoApiError as e:
	if e.http_status_code in [502, 503]:
	soft_error("Service availability error while polling job status: {e}".format(e=e))
	continue

We might want increase the scope of this soft errors functionality