Fix tests

Author: swissspidy

features/plugin-check-update.feature

@@ -98,5 +98,5 @@ Feature: Check for plugin updates
     Then the return code should be 1
     And STDERR should contain:
       """
-      Error:
+      Warning: The 'invalid-plugin-name' plugin could not be found.
       """

features/theme-check-update.feature

@@ -96,5 +96,5 @@ Feature: Check for theme updates
     Then the return code should be 1
     And STDERR should contain:
       """
-      Error:
+      Warning: The 'invalid-theme-name' theme could not be found.
       """