feat(guard): Add optional message.

source/let/guard-spec.ts

@@ -37,4 +37,21 @@ describe("let/guard", () => {
         m.expect(destination).toBeObservable(expected);
         m.expect(source).toHaveSubscriptions(subs);
     }));
+
+    it("should support an error messge", marbles((m) => {
+
+        const values = { a: 1, b: 2, c: "three" };
+        const message = "Not a number";
+
+        const source =   m.cold("-a-b-c-|", values);
+        const subs =            "^----!";
+        const expected = m.cold("-a-b-#", values, new Error(message));
+
+        const destination = source.let(guard(
+            (value): value is number => typeof value === "number",
+            message
+        ));
+        m.expect(destination).toBeObservable(expected);
+        m.expect(source).toHaveSubscriptions(subs);
+    }));
 });

source/let/guard.ts

@@ -8,15 +8,18 @@ import { Observable } from "rxjs/Observable";
 
 import "rxjs/add/operator/map";
 
-export function guard<T, R extends T>(guard: (value: T) => value is R): (source: Observable<T>) => Observable<R> {
+export function guard<T, R extends T>(
+    guard: (value: T) => value is R,
+    message?: string
+): (source: Observable<T>) => Observable<R> {
 
     return (source: Observable<T>) => source.map((value) => {
 
         if (guard(value)) {
             return value as R;
         }
 
-        const error = new Error("Guard rejection.");
+        const error = new Error(message || "Guard rejection.");
         error["value"] = value;
         throw error;
     });