Better error messages for incorrect form actions

[eltigerchino]

Discussed in #11865

^{Originally posted by thebjorn February 18, 2024}
I just got this error:

SvelteKitError: POST method not allowed. No actions exist for this page

I was trying to post from /my/special/page to /logout?next=/goodbye, so it's not exactly clear what "this page" means in this context, especially if you're not experienced (e.g. does it mean a +..page.. file, a path/url, or a route..?).

What can be improved:

1. give me all the information the error handler has - or can gather (it's already an error so there shouldn't be any concerns about using cycles to gather input).
2. give me an error number (SvelteKitError[E042]: ...) that I can look up to find more information (or even make the error number a link to the docs?)

[mig-hub]

Not sure it helps at all but I experienced issues like this when using a 307 redirect inside the action. Then worked with a 302.