message: Prevent deleting databases that have attached tables
  * Prevent deleting databases that have attached tables
  Similar to logic preventing deleting tables that have charts attached.
  * Addressing comments
diff: @@ -256,6 +256,13 @@ class DatabaseView(SupersetModelView, DeleteMixin, YamlExportMixin): # noqa
  def pre_update(self, db):
  self.pre_add(db)
  + def pre_delete(self, obj):
  + if obj.tables:
  + raise SupersetException(Markup(
  + 'Cannot delete a database that has tables attached. '
  + "Here's the list of associated tables: " +
  + ', '.join('{}'.format(o) for o in obj.tables)))
  +
  def _delete(self, pk):
  DeleteMixin._delete(self, pk)
  