message: `export` if package is not built, fail and suggest to `install`
diff: @@ -83,6 +83,16 @@ namespace vcpkg::Commands::Export
  System::println(System::Color::warning, "Additional packages (*) need to be exported to complete this operation.");
  }
  + auto it = group_by_plan_type.find(ExportPlanType::PORT_AVAILABLE_BUT_NOT_BUILT);
  + if (it != group_by_plan_type.cend() && !it->second.empty())
  + {
  + System::println(System::Color::error, "There are packages that have not been built.");
  + auto s = Strings::join(" ", specs, [](const PackageSpec& spec) { return spec.to_string(); });
  + System::println("To build them, run:\n"
  + " vcpkg install %s", s);
  + Checks::exit_fail(VCPKG_LINE_INFO);
  + }
  +
  if (dryRun)
  {
  Checks::exit_success(VCPKG_LINE_INFO);
  @@ -97,6 +107,11 @@ namespace vcpkg::Commands::Export
  // execute the plan
  for (const ExportPlanAction& action : export_plan)
  {
  + if (action.plan_type != ExportPlanType::ALREADY_BUILT)
  + {
  + Checks::unreachable(VCPKG_LINE_INFO);
  + }
  +
  const std::string display_name = action.spec.to_string();
  System::println("Exporting package %s... ", display_name);
  