SplashMixin: improve the package list loading flow

[anttimaki]

• Handle problems in making the API request and saving the response to IndexedDB by silently logging the errors instead of hanging the process with unhandled exceptions. These errors aren't fatal if reading the mod list from cache does succeed
• Handle problems in reading the mod list from IndexedDB into Vuex by showing an error modal instead of hanging the process. If nothing else, the error message might result in easier investigation of issues reported on Discord
• Change the logic so the flow proceeds to profiles screen only if loading the mod list into Vuex resulted in no errors, and the mod list contains mods. An exception to this is that empty mod list is allowed if the API request actually returned an empty list. This is a bit sketchy since we have no way for telling a part "no response has been cached" and "response with empty list is cached"
• If the flow doesn't proceed to profiles screen, the "offline mode" is shown, but this is a bit misnomer now since it's shown even if an API request is made successfully, but processing the result fails e.g. due to IndexedDB error. I feel this is good enough as it is since it allows debugging the rare error situations, and rethinking the whole thing might be too much work
• User may also end up in the "offline mode" if the mod list is successfully loaded into Vuex but e.g. checking deprecations fail. They will then see the online mod list if they proceed, which may be a bit confusing but not a big issue
• Unified the status texts to use loadingText subtitle rather than update the main title so often