maint: don't spam the logs with filtered-out filenames

[kentquirk]

Which problem is this PR solving?

• 324 identified log spam as an issue with the filters. this fixes that part of that issue, but does NOT fix the issue itself.

Short description of the changes

• Keep a map of files we've already complained about, and don't complain again.

[kentquirk]

Any tests possible in tailer_test.go?

I added a test that checks that a file was added to the map appropriately, but can't test the logging directly.