Improve error message

src/csharp/Grpc.Core.Api/ChannelCredentials.cs

@@ -104,7 +104,7 @@ namespace Grpc.Core
 
                 if (!channelCredentials.IsComposable)
                 {
-                    throw new ArgumentException(string.Format("{0} can't be composed with supplied CallCredentials. Secure credentials like SslCredentials must be used.", channelCredentials.GetType().Name));
+                    throw new ArgumentException(string.Format("CallCredentials can't be composed with {0}. CallCredentials must be used with secure channel credentials like SslCredentials.", channelCredentials.GetType().Name));
                 }
             }
 

src/csharp/Grpc.Core.Tests/ChannelCredentialsTest.cs

@@ -41,7 +41,7 @@ namespace Grpc.Core.Tests
 
             // forbid composing non-composable
             var ex = Assert.Throws(typeof(ArgumentException), () => ChannelCredentials.Create(new FakeChannelCredentials(false), new FakeCallCredentials()));
-            Assert.AreEqual("FakeChannelCredentials can't be composed with supplied CallCredentials. Secure credentials like SslCredentials must be used.", ex.Message);
+            Assert.AreEqual("CallCredentials can't be composed with FakeChannelCredentials. CallCredentials must be used with secure channel credentials like SslCredentials.", ex.Message);
         }
 
         [Test]