Return match/reason from `detectors.IsKnownFalsePositive`

[rgmz]

Description:

Recently, there have been several confusing bugs related to false-positive filtering. In each case, determining the cause of this has required manually adding print statements in order to see why.

2643 added a log statement when a result is filtered, however, it doesn't include the reason why.

This updates the method to include the specific match/reason behind it returning true.

Checklist:

• [x] Tests passing (make test-community)?
• [x] Lint passing (make lint this requires golangci-lint)?

[ahrav]

thanks for this! it looks good to me but i want to give @ahrav an opportunity to hold this merge off until he's done fiddling with all the engine code he was planning to fiddle with

I think we can go ahead and merge this. I'll adjust my PR to deal with the merge conflicts, it shouldn't be too bad. Thanks @rgmz