Add Fiber.try().

test/fiber/error.wren

@@ -0,0 +1,7 @@
+var fiber = new Fiber {
+  "s".unknown
+}
+
+IO.print(fiber.error) // expect: null
+IO.print(fiber.try)   // expect: String does not implement method 'unknown'.
+IO.print(fiber.error) // expect: String does not implement method 'unknown'.

test/fiber/is_done_after_error.wren

@@ -0,0 +1,6 @@
+var fiber = new Fiber {
+  "s".unknown
+}
+
+fiber.try
+IO.print(fiber.isDone) // expect: true

test/fiber/try.wren

@@ -0,0 +1,10 @@
+var fiber = new Fiber {
+  IO.print("before")
+  true.unknownMethod
+  IO.print("after")
+}
+
+IO.print(fiber.try)
+// expect: before
+// expect: Bool does not implement method 'unknownMethod'.
+IO.print("after try") // expect: after try

test/fiber/try_direct_reenter.wren

@@ -0,0 +1,7 @@
+var fiber
+
+fiber = new Fiber {
+  fiber.try // expect runtime error: Fiber has already been called.
+}
+
+fiber.call

test/fiber/try_indirect_reenter.wren

@@ -0,0 +1,12 @@
+var a
+var b
+
+a = new Fiber {
+  b.try // expect runtime error: Fiber has already been called.
+}
+
+b = new Fiber {
+  a.call
+}
+
+b.call

test/fiber/try_when_done.wren

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

test/fiber/try_without_error.wren

@@ -0,0 +1,8 @@
+var fiber = new Fiber {
+  IO.print("fiber")
+}
+
+IO.print("before")  // expect: before
+IO.print(fiber.try) // expect: fiber
+                    // expect: null
+IO.print("after")   // expect: after