github swallows errors even if fetching issues fails

[seanstory]

part of #2393

Description

The github connector is currently just logging a warning instead of propagating an error, if something goes wrong while fetching issues. See:

connectors/connectors/sources/github.py

Lines 1373 to 1377 in 8c8d4a3

	except Exception as exception:
	self._logger.warning(
	f"Something went wrong while fetching the repository. Exception: {exception}",
	exc_info=True,
	)

This error should probably be re-raised if it's not something that can be re-tried. If the github connector cannot sync any issues at all, the sync should fail. If it's just a single issue or two, that's probably ok for a WARN message.

Acceptance Criteria

• github connector fails syncs if no issues can be synced
• github connector attempts retries for errors that look transient
• errors with limited scope (single documents, or edge cases) can be moved past with sufficient WARN logging