Add missing error-handling methods

* also added the inspect method (https://github.com/cujojs/when/blob/master/docs/api.md#inspect)

when/when.d.ts

@@ -55,11 +55,25 @@ declare module When {
     }
 
     interface Promise<T> {
+        catch<U>(onRejected?: (reason: any) => Promise<U>): Promise<U>;
+        catch<U>(onRejected?: (reason: any) => U): Promise<U>;
+
         ensure(onFulfilledOrRejected: Function): Promise<T>;
 
+        inspect(): Snapshot<T>;
+
+        otherwise<U>(onRejected?: (reason: any) => Promise<U>): Promise<U>;
+        otherwise<U>(onRejected?: (reason: any) => U): Promise<U>;
+
         then<U>(onFulfilled: (value: T) => Promise<U>, onRejected?: (reason: any) => Promise<U>, onProgress?: (update: any) => void): Promise<U>;
         then<U>(onFulfilled: (value: T) => Promise<U>, onRejected?: (reason: any) => U, onProgress?: (update: any) => void): Promise<U>;
         then<U>(onFulfilled: (value: T) => U, onRejected?: (reason: any) => Promise<U>, onProgress?: (update: any) => void): Promise<U>;
         then<U>(onFulfilled: (value: T) => U, onRejected?: (reason: any) => U, onProgress?: (update: any) => void): Promise<U>;
     }
+
+    interface Snapshot<T> {
+        state: string;
+        value?: T;
+        reason?: any;
+    }
 }