Fix IF implementation to comply with Excel behavior

[frantzmiccoli]

This is:

- [x] a bugfix
- [ ] a new feature

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see /~https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

IF implementation compliant with Excel behavior:

=IF(NA(), 'whatever1', 'whatever2')

was not returning #N/A. Now it does.

[rolandsusans]

Nice catch, checked the result of this example - IF function on my excel
=IF(NA(),1,0) returns NA().
This fix LGTM !

[frantzmiccoli]

Now uses Functions::isError as suggested by @MarkBaker on #1167