Make Backdrop misconfiguration messages friendlier

I submitted this yesterday as an initiative proposal here but perhaps it can be satisfied merely as a feature. I'll copy-and-paste whatever is relevant from that here.

Description of the need

When Backdrop fails to bootstrap, it produces a very unfriendly message, like this one complaining about problems with the configuration directory:

To a developer familiar with the way Backdrop works, this makes sense, but there are a lot situations where this message will be shown to an unsuspecting user -- perhaps an administrator that renamed something, or an inexperienced developer trying to move a site to a different host, or perhaps the hosting provider changed something.

Proposed solution

Do we have to throw them to the wolves? They're still running our code. So what if we can't find the configuration file, there's no need to throw a tantrum! Let's just tell them what we need.

Perhaps something like

This Backdrop site is unable to finish loading, because a needed configuration file could not be found. Information to try to help you resolve that is printed below, in small friendly type.

(I realize we need to get them the information! I'm mostly proposing a different presentation, so that they're more hopeful that they'll be able to get their site working again.)

Draft of feature description for Press Release (1 paragraph at most)

While nobody likes to see a message telling them that a site isn't working, there's no need to shock them unnecessarily. We've redesigned several error screens to project a sense of calm and security, while still conveying the necessary information.