VS: add missing label in C# project-build events

Source/cmLocalVisualStudioGenerator.cxx

@@ -243,3 +243,20 @@ std::string cmLocalVisualStudioGenerator::ConstructScript(
 
   return script;
 }
+
+std::string cmLocalVisualStudioGenerator::FinishConstructScript(
+  IsManaged isManaged, const std::string& newline)
+{
+  bool useLocal = this->CustomCommandUseLocal();
+
+  // Store the script in a string.
+  std::string script;
+
+  if (useLocal && isManaged == IsManaged::Yes) {
+    // These aren't generated by default for C# projects.
+    script += newline;
+    script += this->GetReportErrorLabel();
+  }
+
+  return script;
+}