Revamp how runtime errors and fiber switching is handled.

- Add Fiber.transferError(_).
- Primitives place runtime errors directly in the fiber instead of on
  the stack.
- Primitives that change fibers set it directly in the VM.
- Allow a fiber's error to be any object (except null).

test/core/fiber/abort_not_string.wren

@@ -0,0 +1,7 @@
+var fiber = Fiber.new {
+  Fiber.abort(123)
+}
+
+System.print(fiber.try()) // expect: 123
+System.print(fiber.isDone) // expect: true
+System.print(fiber.error) // expect: 123

test/core/fiber/abort_null.wren

@@ -0,0 +1,9 @@
+var fiber = Fiber.new {
+  Fiber.abort(null)
+  System.print("get here") // expect: get here
+  Fiber.yield("value")
+}
+
+System.print(fiber.try()) // expect: value
+System.print(fiber.isDone) // expect: false
+System.print(fiber.error) // expect: null

test/core/fiber/abort_wrong_arg_type.wren

@@ -1 +0,0 @@
-Fiber.abort(123) // expect runtime error: Error message must be a string.

test/core/fiber/transfer_error.wren

@@ -0,0 +1,15 @@
+var A = Fiber.new {
+  System.print("transferred to A")
+  B.transferError("error!")
+}
+
+var B = Fiber.new {
+  System.print("started B")
+  A.transfer()
+  System.print("should not get here")
+}
+
+B.try()
+// expect: started B
+// expect: transferred to A
+System.print(B.error) // expect: error!

test/core/fiber/transfer_error_not_string.wren

@@ -0,0 +1,10 @@
+var A = Fiber.new {
+  B.transferError(123)
+}
+
+var B = Fiber.new {
+  A.transfer()
+}
+
+B.try()
+System.print(B.error) // expect: 123