Updates to API 404 responses

[jolheiser]

This PR will address #6052

So far it handles API 404s.
Instead of a blank 404, it returns a JSON response with a message of "Not Found" and a documentation_url pointing to the basic swagger endpoint. e.g. https://try.gitea.io/api/swagger

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@d10a668). Click here to learn what that means.
The diff coverage is 18.18%.

@@            Coverage Diff            @@
##             master    #6077   +/-   ##
=========================================
  Coverage          ?   38.87%           
=========================================
  Files             ?      365           
  Lines             ?    51341           
  Branches          ?        0           
=========================================
  Hits              ?    19957           
  Misses            ?    28511           
  Partials          ?     2873

Impacted Files	Coverage Δ
routers/api/v1/org/hook.go	0% <0%> (ø)
routers/api/v1/repo/release_attachment.go	0% <0%> (ø)
routers/api/v1/repo/release.go	44.94% <0%> (ø)
routers/api/v1/repo/issue_tracked_time.go	0% <0%> (ø)
routers/api/v1/repo/issue_label.go	29.52% <0%> (ø)
routers/api/v1/org/member.go	0% <0%> (ø)
routers/api/v1/repo/issue.go	23.46% <0%> (ø)
routers/api/v1/repo/hook.go	18.77% <0%> (ø)
routers/api/v1/user/star.go	0% <0%> (ø)
routers/api/v1/user/key.go	64.35% <0%> (ø)
... and 20 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d10a668...2ce89fc. Read the comment docs.

[zeripath]

I'd prefer a path.Join in there instead of fmt.Sprintf

[lafriks]

It should probably moved to some kind of helper method as all 404 errors (also ones returned from methods where record by id is not found etc) should return same imho

[jolheiser]

For the record, I had planned on fixing both things in the original issue, but perhaps this should be a PR on its own just for API 404s. 😄

[techknowlogick]

This PR is marked as WIP, is it still WIP?

[jolheiser]

I guess it depends on whether we want the other sub-issue in the original issue fixed in this PR or a separate one. If not, this is complete for API 404s.

[jolheiser]

No comment on the original issue, so I will count this PR as just fixes to API 404s.