Error pages for 500/504/etc.

[Discookie]

(Moved from #173)

The site currently doesn't have proper error pages for most errors.
I think instead of the raw HTTP codes/one-liners, it should give a bit of explanation:

• for 503/504 one that would explain that the servers are overloaded
• for 500 one that would point them to make an issue here
• etc etc.

I know some errors (like 404) do have their own pages, but I think all but the worst errors should be caught before the user sees them.

[omarroth]

Proposed errors:

For 503/504: Unfortunately this is an error provided by the host, I don't think there's much I can do to change it.
For 500,other fatal errors: "Looks like you've found a bug in Invidious. Feel free to open an issue here or send an email to omarroth@protonmail.com"
For 404: YouTube actually redirects to / instead of showing 404, I'd be open to doing that instead

I think most errors are caught before the user sees them, although sometimes an error can crop up where I didn't expect it, so there's no handling for it. I think those are getting fewer, as I've tried to make errors non-fatal (leaving a video out of search if Invidious can't extract info from it for example, or leaving channels empty instead of 500).

[omarroth]

Added proposed error messages with dfd17bd.