Clean up image filter and cluster filter isLoading property and error message

On the vulnerability report, for the image filter and cluster filter, we should do the following:

1. The isLoading property looks like a boolean, but is actually a loading key used by GraphQL. We should rename this to loading to avoid confusion.

2. The loading error message is stored in constants.js, but it's only used in one place, in each respective filter. We should move it into $options.i18n instead.