Add a `verb` string parameter to the `runFiber` helper function so that
`Fiber.try()` produces consistent error messages like "Cannot try a finished
fiber." instead of the previous generic fallback. Update all call sites
(`fiber_call`, `fiber_call1`, `fiber_transfer`, `fiber_transfer1`) to pass
the appropriate verb string, and add a new test case `try_error.wren`
verifying the error message for trying an aborted fiber.
