BackfillJob eventually fails tasks when dependencies aren't met

[jlowin]

See #1225 (comment)

When dependencies are unmet, the command exists successfully (as in, without error or return code). The executor marks it as a success. But the task has no status, because it never ran. This triggers the "airflow run command failed to report an error" which eventually fails the task.

This issue will be fixed as part of the resolution for #1225, this issue is for tracking it.