NUTCH-3164 Catch specific exceptions in CrawlDbFilter so plugin errors no longer silently drop URLs

[prakharchaube]

Summary

Both catch blocks in CrawlDbFilter.map() caught generic Exception and set url = null, which silently dropped URLs both for legitimate filtering reasons and for plugin
programming errors (NPE, etc.). The latter masked plugin bugs as ordinary filtering decisions.

Changes

Normalizer block

• MalformedURLException — the only legitimate reason to drop. Tracked via ErrorTracker (ErrorType.URL) and no longer increments urlsFilteredCounter, which conflated
  filtering with malformed input.
• RuntimeException — logged at ERROR and tracked, URL is not dropped so plugin bugs do not silently delete data.

Filter block

• URLFilterException — per the URLFilter contract, reserved for internal filter failures (rejection is signaled by returning null). Logged at ERROR and tracked, URL is
  not dropped.
• RuntimeException — same handling as above.

All error paths now use ErrorTracker for categorized counters and log at ERROR rather than WARN, per @lewismc's recommendation in the JIRA
discussion.

JIRA

NUTCH-3164

Test plan

• ant compile passes locally
• No new unit tests in this PR; happy to add one if reviewers want a mock plugin throwing each exception type (flag as follow-up otherwise)

Out of scope (separate tickets if desired)

• Same catch (Exception) pattern exists in Injector.filterNormalize and ~20 other call sites of normalizers.normalize / filters.filter — per lewismc comment, those
  should be rolled out separately.
• Whether to track normalized URLs as a metric system-wide.