Unify Fiber.try() with the other methods to execute fibers.

test/core/fiber/try_error.wren

@@ -0,0 +1,7 @@
+var fiber = Fiber.new {
+  System.print("try")
+  Fiber.abort("err")
+}
+
+fiber.try() // expect: try
+fiber.try() // expect runtime error: Cannot try an aborted fiber.