Fix broken pixelsNeededForErrorMessages method

- Fixed broken logic in pixelsNeededForErrorMessages.  This method
  incorrectly calculated the height of the error message box.
- Added check to ensure that distortMaxValuesBy can never return a
  negative scale or cause a division by zero exception.  Both were
  previously possible if the error/pointFlags/x2-labels had a combined
  height that was larger than the height of the graph.
- Extracted code to inject the error message box to guarantee that the
  same classes and styling are applied both when Elroi inject/removes
  to garner size in pixelsNeededForErrorMessages and when Elroi
  actually inject the visible error message.
- Replaced instances where the visibility-hidden class could be used
  but wasn't.
Merge pull request #19 from gsmith85/fix-broken-pixelsNeededForErrorMessages-method

Fix broken pixelsNeededForErrorMessages method