package: Fix showing error during pre/post install

When an ActionError is raised during pre_install or on_install action,
the error is not being shown.  Instead an error is being raised.  This
fixes that.

plinth/package.py

@@ -263,7 +263,8 @@ def _should_show_install_view(request, package_names):
         return False
     else:
         messages.error(request, _('Error installing packages: {details}')
-                       .format(details=exception.error_string))
+                       .format(details=getattr(exception, 'error_string',
+                                               str(exception))))
         return True