ErrorState (And related components): expose headingLevel prop

[nicolethoen]

There might be times (for accessibilty purposes) that the heading level of the title in the ErrorState will need to be configurable.

Can we add a prop which would allow the consumer to provide their own headingLevel to conform to with WCAG rule and avoid skipping heading levels: https://www.w3.org/WAI/tutorials/page-structure/headings/