Add fail-fast flag to src batch

Created by: gabtorre

This adds a new --fail-fast flag to the src batch commands. When enabled, any errors encountered while executing steps in a repository will stop the execution of the whole batch spec. The flag is off by default, so existing behavior is preserved.

• A new CancelableContext type was added to support cancellation with errors. The batch commands now wrap the context with this to enable fail-fast behavior.
• Updated contextCancelOnInterrupt to support cancellation with error causes.
• Added error check in executeBatchSpec to surface fail-fast cancellation errors.
• Errors now propagate up by calling the cancel context's Cause()
• Minor code reorganizations around log file handling to return logs if -fail-fast and -keep-logs are used together.

Test plan

Updated tests.