Added RichErrors to Ownable

contracts/utils/contracts/src/MixinOwnableRichErrors.sol

@@ -0,0 +1,28 @@
+pragma solidity ^0.5.9;
+
+import "./RichErrors.sol";
+
+
+contract MixinOwnableRichErrors is
+    RichErrors
+{
+    // bytes4(keccak256("OnlyOwnerError(address,address)"))
+    bytes4 internal constant ONLY_OWNER_SELECTOR =
+        0x1de45ad1;
+
+    // solhint-disable func-name-mixedcase
+    function OnlyOwnerError(
+        address sender,
+        address owner
+    )
+        internal
+        pure
+        returns (bytes memory)
+    {
+        return abi.encodeWithSelector(
+            ONLY_OWNER_SELECTOR,
+            sender,
+            owner
+        );
+    }
+}

contracts/utils/contracts/src/Ownable.sol

@@ -1,9 +1,11 @@
 pragma solidity ^0.5.9;
 
 import "./interfaces/IOwnable.sol";
+import "./MixinOwnableRichErrors.sol";
 
 
 contract Ownable is
+    MixinOwnableRichErrors,
     IOwnable
 {
     address public owner;
@@ -15,10 +17,12 @@ contract Ownable is
     }
 
     modifier onlyOwner() {
-        require(
-            msg.sender == owner,
-            "ONLY_CONTRACT_OWNER"
-        );
+        if (msg.sender != owner) {
+            _rrevert(OnlyOwnerError(
+                msg.sender,
+                owner
+            ));
+        }
         _;
     }