Add additional checks to Vector class methods (#1721)

* Add additional checks to `Vector` class methods

* #1618 (comment)
* Change `ensureCapacity` to adjust new capacity by `increment`, thus simplifying code which uses it
* Add `bool` return value to:
	`addElement()` - so that `add()` return value isn't meaningless
	`ensureCapacity()`
	`setSize()`
	`insertElementAt()`
	`setElementAt()`

* Apply coding style and fix use of parentheses