Update documentation.

docs/common-patterns.rst

@@ -147,7 +147,10 @@ restrictions highly readable.
         // a certain address.
         modifier onlyBy(address _account)
         {
-            require(msg.sender == _account);
+            require(
+                msg.sender == _account,
+                "Sender not authorized."
+            );
             // Do not forget the "_;"! It will
             // be replaced by the actual function
             // body when the modifier is used.
@@ -164,7 +167,10 @@ restrictions highly readable.
         }
 
         modifier onlyAfter(uint _time) {
-            require(now >= _time);
+            require(
+                now >= _time,
+                "Function called too early."
+            );
             _;
         }
 
@@ -186,7 +192,10 @@ restrictions highly readable.
         // This was dangerous before Solidity version 0.4.0,
         // where it was possible to skip the part after `_;`.
         modifier costs(uint _amount) {
-            require(msg.value >= _amount);
+            require(
+                msg.value >= _amount,
+                "Not enough Ether provided."
+            );
             _;
             if (msg.value > _amount)
                 msg.sender.send(msg.value - _amount);
@@ -290,7 +299,10 @@ function finishes.
         uint public creationTime = now;
 
         modifier atStage(Stages _stage) {
-            require(stage == _stage);
+            require(
+                stage == _stage,
+                "Function cannot be called at this time."
+            );
             _;
         }