Hassio: Allow Add-On To Provide A More Helpful Hint Than "502 Bad Gateway"

[GfEW]

Since giving genuine "first time user's feedback" is one of the rare endeavours that get harder with experience, I've brought this one up right away: hassio-addons/addon-ssh#662 .
Please see there for the rationale and suggested solution (addressing a particular newbie struggle with a non-appearing webUI that in hindsight, feels unnecessarly time consuming and could easily be fast forwareded by an appropriate help text).

As it turns out though, the issue can't be resolved at the add-on level, because neither the "502 Bad Gateway" error page nor the "The add-on seems to not be ready, it might still be starting, Do you want to try again?" popup are customizable.

The popup may be seen as an improvement over the mere 502 already, but its general wording cannot adequately address addon-specifics like the non-obvious strict initial configuration requirement, nor the huge startup delay, of add-on ssh.

Therefore, I'd like to suggest one of the following improvements:

• (a) add-ons should be allowed to provide a custom popup message in place of the default text that reads "The add-on seems to not be ready, it might still be starting, Do you want to try again?"
• (b) alternatively, add-ons should be enabled to provide a custom error page that is shown instead of the default "502 Bad Gateway" as long as the add-on's webUI isn't ready.

I wouldn't be surprised if customized hints were beneficial to first time users' experience with other add-ons, as well.

What's more, such entry level difficulties are notoriously underreported, as they largely affect users who don't file bug reports (even when giving up entirely), and the ones who manage to find a solution then tend to focus on utilizing it, forgetting about their initial struggles. Please bear this in mind when weighing the merits vs. costs of implementation.

(Also, please feel free to move this issue to a more suitable place if "frontend/accessibility" doesn't feel right.)