Make blocks throwable

Sources/Then/Then.swift

@@ -36,9 +36,9 @@ extension Then where Self: Any {
   ///       $0.origin.x = 10
   ///       $0.size.width = 100
   ///     }
-  public func with(_ block: (inout Self) -> Void) -> Self {
+  public func with(_ block: (inout Self) throws -> Void) rethrows -> Self {
     var copy = self
-    block(&copy)
+    try block(&copy)
     return copy
   }
 
@@ -49,8 +49,8 @@ extension Then where Self: Any {
   ///       $0.set("devxoul@gmail.com", forKey: "email")
   ///       $0.synchronize()
   ///     }
-  public func `do`(_ block: (Self) -> Void) {
-    block(self)
+  public func `do`(_ block: (Self) throws -> Void) rethrows {
+    try block(self)
   }
 
 }
@@ -64,8 +64,8 @@ extension Then where Self: AnyObject {
   ///       $0.textColor = UIColor.blackColor()
   ///       $0.text = "Hello, World!"
   ///     }
-  public func then(_ block: (Self) -> Void) -> Self {
-    block(self)
+  public func then(_ block: (Self) throws -> Void) rethrows -> Self {
+    try block(self)
     return self
   }