Handle errors with fiber usage.

test/fiber/is_done.wren

@@ -0,0 +1,11 @@
+var fiber = Fiber.create(fn {
+  IO.print("1")
+  Fiber.yield
+  IO.print("2")
+})
+
+IO.print(fiber.isDone)  // expect: false
+fiber.run               // expect: 1
+IO.print(fiber.isDone)  // expect: false
+fiber.run               // expect: 2
+IO.print(fiber.isDone)  // expect: true

test/fiber/run_when_done.wren

@@ -0,0 +1,6 @@
+var fiber = Fiber.create(fn {
+  IO.print("run")
+})
+
+fiber.run // expect: run
+fiber.run // expect runtime error: Cannot run a finished fiber.

test/fiber/run_with_value_when_done.wren

@@ -0,0 +1,6 @@
+var fiber = Fiber.create(fn {
+  IO.print("run")
+})
+
+fiber.run(1) // expect: run
+fiber.run(2) // expect runtime error: Cannot run a finished fiber.

test/fiber/yield_from_main.wren

@@ -0,0 +1 @@
+Fiber.yield // expect runtime error: No fiber to yield to.

test/fiber/yield_with_value_from_main.wren

@@ -0,0 +1 @@
+Fiber.yield(1) // expect runtime error: No fiber to yield to.