Centralized error handling into new Result class:

Executive:
- Change StackOperator::operator() to return result
- Parse new Result for info
- Add messages to Result to indicate quit

BinaryMathOperator:
StackItem:
StackManipulator:
StackOperator:
VariableManipulator:
- Change StackOperator::operator() to return result

Stack:
- Changed errors to Result
- Add messages to add error messages at current or specified position to manage errors uniformly
- Change argument to StackIteratorPimpl from iterator to stack so we can check before dereference

Error:
- Removed no longer necessary atPosition()

StackReadingExample:
- Refactor to show returning Result on failure
- Also, setup item 5 to fail when the example is run

Helpers:
- Add method to test expected Result messages and positionMessages

MathOperators:
StackManipulators:
- Update testing for new Result