Add wrenAbortFiber().

Thanks, @underscorediscovery!

src/vm/wren_value.h

@@ -513,7 +513,7 @@ typedef struct
 // S[NaN      ]1---------------------------------------------------
 //
 // For singleton values, we just enumerate the different values. We'll use the
-// low three bits of the mantissa for that, and only need a couple:
+// low bits of the mantissa for that, and only need a few:
 //
 //                                                 3 Type bits--v
 // 0[NaN      ]1------------------------------------------------[T]

src/vm/wren_vm.c

@@ -927,6 +927,7 @@ static WrenInterpretResult runInterpreter(WrenVM* vm, register ObjFiber* fiber)
 
         case METHOD_FOREIGN:
           callForeign(vm, fiber, method->fn.foreign, numArgs);
+          if (!IS_NULL(fiber->error)) RUNTIME_ERROR();
           break;
 
         case METHOD_FN_CALL:
@@ -1660,3 +1661,9 @@ void wrenGetVariable(WrenVM* vm, const char* module, const char* name,
   
   setSlot(vm, slot, moduleObj->variables.data[variableSlot]);
 }
+
+void wrenAbortFiber(WrenVM* vm, int slot)
+{
+  validateApiSlot(vm, slot);
+  vm->fiber->error = vm->apiStack[slot];
+}