Improve error on manual run when connection with Forge lost

[zc-devs]

Component

server

Describe the bug

If connection broken while you are running pipeline manually, you'll get unknown error and panic on Server.

Steps to reproduce

1. Run Forge
2. Run Woodpecker server, log in
3. Open some repository
4. Click Run pipeline
5. Turn off Forge / scale down to 0 / disallow WP-Forge connection via firewall
6. Close Trigger a manual pipeline run dialog by pressing Run pipeline button
7. See unknown error and not closed overlay
   Error
8. Check server logs and find panic recovered
   server-panic.log

Expected behavior

1. Should not be panic on server
2. As dialog closes, overlay should disappear also
3. More meaningful error than unknown

System Info

`2.7.0`, Gitea

Additional context

No response

Validations

• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Checked that the bug isn't fixed in the next version already [https://woodpecker-ci.org/faq#which-version-of-woodpecker-should-i-use]

[zc-devs]

next-bbdeb4622c

While Should not be panic on server was fixed,

As dialog closes, overlay should disappear also
More meaningful error than unknown

are not addressed. Still unknown error (HTTP 500) and overlay doesn't close automatically.