Enforce use of `throw` stmt in early error tests

Previously, test consumers were encouraged to insert a `throw` statement
as the first statement of tests for early errors. This recommendation
made tests harder to consume, and as an optional transformation,
consumers may have ignored it or simply been unaware it was made. By
explicitly including such a `throw` statement, the tests become more
literal, making them easier to consume and more transparent in their
expectations.

Document expectation for all tests for early errors to include an
explicit `throw` statement. Extend linting script to verify that
contributors are automatically notified of violations and to ensure that
future contributions satisfy this expectation.