No more calls to fatalError

Fixes #537

In this change

• The constraint maker no longer calls fatalError but emits a ConstraintMakerEditable with an error inside
• The prepareConstraints(item:closure:) method checks for errors and calls the ConstraintMakerErrorHandler
• The ConstraintMakerErrorHandler calls to assertionFailure instead of fatalError to prevent production crashers. Its behaviour can be globally overridden
• All calls to fatalError have been changed to call assertionFailure
• An unneeded force unwrap has been resolved
• Make layoutConstraintItem non optional because its optionality is not needed and not handled well