Update error message non-matching sheet names.

src/Form/ExcelImporterForm.php

@@ -85,7 +85,7 @@ class ExcelImporterForm extends FormBase {
         }
 
         if ($valid_sheets_count == 0) {
-          $form_state->setErrorByName('excel_file', t('The file needs to contains at least one sheet with a valid content type name.'));
+          $form_state->setErrorByName('excel_file', t('The file needs to contains at least one sheet with a valid content type name (machine name). Please check if the sheet names of your file match with the content types allowed in the Excel Importer administration settings (admin/config/content/excel_importer).'));
         }
       }
       catch (Exception $e) {