db: manual compactions that are internally cancelled should be retried

[sumeerbhola]

We expose any error in a manual compaction to the client that requested the manual compaction. This was appropriate when an error may have represented something serious and non transient, which is how we had originally implemented compactions.

However, we now have internal cancellation

pebble/compaction.go

Line 176 in 1157615

cancel atomic.Bool

which can happen due to an ingest (or flushable ingest) that excises or splits files involved in the compaction. An error due to an internal cancellation should be retried.

Jira issue: PEBBLE-335