SplitOn: improve error message

Fixes #866. This provides a more informative error, saying _which_ splitOn column couldn't be found in the result set.

Dapper/SqlMapper.cs

@@ -1679,7 +1679,7 @@ private static int GetNextSplitDynamic(int startIdx, string splitOn, IDataReader
         {
             if (startIdx == reader.FieldCount)
             {
-                throw MultiMapException(reader);
+                throw MultiMapException(reader, splitOn);
             }
 
             if (splitOn == "*")
@@ -1713,7 +1713,7 @@ private static int GetNextSplit(int startIdx, string splitOn, IDataReader reader
                 }
             }
 
-            throw MultiMapException(reader);
+            throw MultiMapException(reader, splitOn);
         }
 
         private static CacheInfo GetCacheInfo(Identity identity, object exampleParameters, bool addToCache)
@@ -1831,16 +1831,18 @@ private static void PassByPosition(IDbCommand cmd)
             return reader => handler.Parse(type, reader.GetValue(startBound));
         }
 
-        private static Exception MultiMapException(IDataRecord reader)
+        private static Exception MultiMapException(IDataRecord reader, string splitOnColumnName = null)
         {
             bool hasFields = false;
             try { hasFields = reader != null && reader.FieldCount != 0; }
             catch { /* don't throw when trying to throw */ }
             if (hasFields)
             {
-#pragma warning disable CA2208 // Instantiate argument exceptions correctly
-                return new ArgumentException("When using the multi-mapping APIs ensure you set the splitOn param if you have keys other than Id", "splitOn");
-#pragma warning restore CA2208 // Instantiate argument exceptions correctly
+                return new ArgumentException(
+                    string.IsNullOrEmpty(splitOnColumnName)
+                    ? "When using the multi-mapping APIs ensure you set the splitOn param if you have keys other than Id"
+                    : $"Multi-map error: splitOn column '{splitOnColumnName}' was not found - please ensure your splitOn parameter is set and in the correct order", 
+                    "splitOn");
             }
             else
             {

tests/Dapper.Tests/MultiMapTests.cs

@@ -194,6 +194,13 @@ public void TestMultiMapperIsNotConfusedWithUnorderedCols()
             Assert.Equal("a", result.Item2.Name);
         }
 
+        [Fact]
+        public void TestMultiMapperSplitOnError()
+        {
+            var ex = Assert.Throws<ArgumentException>(() => connection.Query<Foo1, Bar1, Tuple<Foo1, Bar1>>("select 1 as Id, 2 as BarId", Tuple.Create, splitOn: "DoesntExist").First());
+            Assert.StartsWith("Multi-map error: splitOn column 'DoesntExist' was not found - please ensure your splitOn parameter is set and in the correct order", ex.Message);
+        }
+
         [Fact]
         public void TestMultiMapDynamic()
         {