Default to div[role="region"], update messages, and add tests

[kmillns]

Addresses:
#8 - Default use of component throws error

• Warns instead of errors and defaults to div[role="region"]

#7 - Assertions for only a tagName or a landmarkRole?

• Updates assert messaging to match use of either div or form with landmarkRole

[kmillns]

@EndangeredMassa was told you should review this to see if we're missing something in the logic.

[MelSumner]

Thank you @kmillns!