Change LintRule#getLintError specification.

- From Optional<LintError> getLintError to List<LintError>
  getLintErrors. This is a breaking change.
Make html dump test VERSION agnostic
Rename LintRulesImpl#isTestMode to #willExitAfterReporting

- This makes it clearer to see what it's actually doing, especially
  since this could be called externally.
jxlint 1.0.2