improve error messaging

Author: mpeddada1

google/cloud/bigtable/internal/data_connection_impl.cc

@@ -155,7 +155,8 @@ class DefaultPartialResultSetReader
       // Throw an error when ExecuteQueryResponse returns metadata.
       if (response.has_metadata()) {
         final_status_ = internal::InternalError(
-            "Response contains unknown type metadata", GCP_ERROR_INFO());
+            "Expected results response, but received: METADATA",
+            GCP_ERROR_INFO());
         operation_context_->PostCall(*context_, final_status_);
         return false;
       }

google/cloud/bigtable/internal/data_connection_impl_test.cc

@@ -3750,7 +3750,7 @@ TEST_F(DataConnectionTest, ExecuteQueryFailureWithSchemaChange) {
   for (auto const& row : row_stream) {
     EXPECT_THAT(row,
                 StatusIs(StatusCode::kInternal,
-                         HasSubstr("Response contains unknown type metadata")));
+                         HasSubstr("Expected results response, but received: METADATA")));
   }
   fake_cq_impl->SimulateCompletion(false);
 }

google/cloud/bigtable/internal/partial_result_set_source.cc

@@ -211,8 +211,9 @@ Status PartialResultSetSource::BufferProtoRows() {
     auto const& proto_values = proto_rows_.values();
 
     if (columns_size == 0 && !proto_values.empty()) {
-      return internal::InternalError("columns cannot be empty",
-                                     GCP_ERROR_INFO());
+      return internal::InternalError(
+          "ProtoRows has values but ResultSetMetadata columns cannot be empty",
+          GCP_ERROR_INFO());
     }
     if (proto_values.size() % columns_size != 0) {
       state_ = State::kFinished;