[Fix #398] Make minimum error level for navigation customizable
Allow nil value for error level
Change default to nil
Document the new option
Improve helper macro for navigation tests
[Fix #485] Merge branch 'minimum-error-level'

* minimum-error-level:
  Improve helper macro for navigation tests
  Document the new option
  Change default to nil
  Allow nil value for error level
  [Fix #398] Make minimum error level for navigation customizable
Update credits, changelog and texinfo manual [#485]