Make error rendering context-aware

[Gusted]

• Currently the error handling in modules/context/context.go always use ctx.HTML(indirectly) to render the error, however it is possible that the context doesn't have any HTML render set(e.g. APIContext).
• Let the context creator provide the error handling, currently web will still use the current functions and API/Package will roughly do the same but using ctx.JSON.
• Implement a dummyRender for the API/Package Context to not see a vague NPE error.

For clarity purposes, this panic was found via:

gitea/routers/api/v1/api.go

Line 152 in 6f3efdf

context.RedirectToUser(ctx.Context, userName, redirectUserID)

->

gitea/modules/context/context.go

Line 144 in 6f3efdf

ctx.ServerError("GetUserByID", err)

->

gitea/modules/context/context.go

Line 305 in 6f3efdf

ctx.HTML(http.StatusInternalServerError, base.TplName("status/500"))

->

gitea/modules/context/context.go

Line 227 in 6f3efdf

if err := ctx.Render.HTML(ctx.Resp, status, string(name), templates.BaseVars().Merge(ctx.Data)); err != nil {

ContextAPI doesn't set the Render field, which caused an NPE.

[lunny]

For a long-term target, APIContent should have a new struct to remove unnecessary fields rather than deriving from Context. The benefit of that is less memory usage per request especially for those big Gitea instances. #16455 should be the first step towards that target. I'm not against this PR but I think this still need to be changed in future.

[6543]

Without looking more into the code, did you make sure normal users dont see the stacktrace in prod mode?

[Gusted]

Stacktraces were never exposed to begin with and conditions were copies (ctrl + f setting.IsProd)

[lunny]

For web routers, sometimes it will return JSON, sometimes return HTML and for RSS it's XML.
I couldn't find it becomes more convenient than explicitly invoking a render function in the router function.

[Gusted]

For web routers, sometimes it will return JSON, sometimes return HTML and for RSS it's XML. I couldn't find it becomes more convenient than explicitly invoking a render function in the router function.

That would require a very broad refactor, which I won't contribute. If that's the consensus to fix this problem, feel free to close my PR.

[wxiaoguang]

We need to decouple the Gitea's contexts first. Otherwise more things are mixed together.

-> Decouple WebContext/APIContext/PrivateContext #24786