feat: unify Fiber.try() error messages with call/transfer by adding verb parameter to runFiber

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.

test/core/fiber/try_done.wren

@@ -0,0 +1,6 @@
+var fiber = Fiber.new {
+  System.print("try")
+}
+
+fiber.try() // expect: try
+fiber.try() // expect runtime error: Cannot try a finished fiber.