rename `App.PageError` to `App.Error`

[Rich-Harris]

closes #6905. This is a breaking change, but I expect it to be very limited in impact.

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[dummdidumm]

Do you want to implement the "try to rename that inside src/app.d.ts during sync" thing I proposed in the issue, or did you decide against it?

[Rich-Harris]

I agree with Dominik that it's probably a slippery slope. I think throwing an error telling the user to change it could be good, it just felt like a lot of effort when considering that we don't enforce a specific location for the user to declare those namespaces

[dummdidumm]

We don't but I guess that 90% of the 5% of people that use this have it inside src/app.d.ts, which is where the starter templates put this, so we could check if that file exists, if yes if it has the app namespace, if yes if it has AppError in there, if yes throw, else do nothing.

[asmajlovicmars]

Not sure if it matters much, but even if PageError is commented in app.d.ts/js, the error will be produced: error when starting dev server: Error: App.PageError has been renamed to App.Error — please update your src/app.d.ts

// app.d.ts
// interface PageError {}

[dummdidumm]

Mhm yeah that's probably very annoying for everyone who doesn't even use it - probably worth expanding the check to see if it's commented out.