Fix #17: View.Do: add TryWith, TryFinally, ReturnFrom

Unfortunately I believe there are no correct semantics
for Using due to Snap's implicit lifetime.

WebSharper.UI.Next/Reactive.fs

@@ -271,6 +271,23 @@ type View =
         let o = Snap.CreateForever x
         V (fun () -> o)
 
+    static member TryWith (f: exn -> View<'T>) (V observe: View<'T>) : View<'T> =
+        View.CreateLazy (fun () ->
+            try
+                observe ()
+            with exn ->
+                let (V obs) = f exn
+                obs ()
+        )
+
+    static member TryFinally (f: unit -> unit) (V observe: View<'T>) : View<'T> =
+        View.CreateLazy (fun () ->
+            try
+                observe ()
+            finally
+                f ()
+        )
+
     static member Sink act (V observe) =
         let rec loop () =
             let sn = observe ()
@@ -380,6 +397,15 @@ type ViewBuilder =
     [<JavaScript; Inline>]
     member b.Return x = View.Const x
 
+    [<JavaScript; Inline>]
+    member b.ReturnFrom(v: View<'T>) = v
+
+    [<JavaScript; Inline>]
+    member b.TryWith(v, f) = View.TryWith f v
+
+    [<JavaScript; Inline>]
+    member b.TryFinally(v, f) = View.TryFinally f v
+
 type View with
     [<JavaScript>]
     static member Do = B