Check context before notifying frontend and scheduler

[justinjung04]

What this PR does:

We have two extra logs when the query times out:

ts=2023-08-29T15:57:55.2976267Z caller=scheduler_processor.go:187 level=info msg="finished request" status_code=499 response_size=95

ts=2023-08-29T15:57:55.297738564Z caller=scheduler_processor.go:211 level=error msg="error notifying frontend about finished query" err="rpc error: code = Canceled desc = context canceled" frontend=10.1.121.156:9095

ts=2023-08-29T15:57:55.297766783Z caller=scheduler_processor.go:163 level=error msg="error notifying scheduler about finished query" err=EOF addr=10.1.126.155:9095

We can check the context before notifying the frontend and scheduler if context error is not nil.

Which issue(s) this PR fixes:
n/a

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[harry671003]

Could you help me understand why the extra logs are a problem?

[justinjung04]

They're not really problematic, just want to remove extra error logs that's not helpful