chore: handle error types consistently (#9873)

Some error types are used inconsistently or wrong:

- `forgejo.org/modules/git.ErrNotExist` is meant to be a value error: <[modules/git/error.go#L23](https://codeberg.org/forgejo/forgejo/src/tag/v13.0.2/modules/git/error.go#L23)>

- `forgejo.org/models/repo.ErrRepoNotExist` is meant to be a value error: <[models/repo/repo.go#L750](https://codeberg.org/forgejo/forgejo/src/tag/v13.0.2/models/repo/repo.go#L750)>

- `errors.Is(logErr, &net.OpError{})` is always `false`: <[services/context/context_response.go#L188](https://codeberg.org/forgejo/forgejo/src/tag/v13.0.2/services/context/context_response.go#L188)>

- `forgejo.org/models/issues.ErrIssueContentHistoryNotExist` is used inconsistently: <[models/issues/content_history.go#L211](https://codeberg.org/forgejo/forgejo/src/tag/v13.0.2/models/issues/content_history.go#L211)>
Decided to use a value, since the structure is small and to be in line with the above errors.

These issued where found with the [errortype](https://codeberg.org/fillmore-labs/errortype) linter and add this to Makefile as part of the linter suite.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/9873
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: Oliver Eikemeier <eikemeier@fillmore-labs.com>
Co-committed-by: Oliver Eikemeier <eikemeier@fillmore-labs.com>

models/issues/content_history.go

@@ -222,7 +222,7 @@ func GetIssueContentHistoryAndPrev(dbCtx context.Context, issueID, id int64) (hi
 		return nil, nil, err
 	} else if !has {
 		log.Error("issue content history does not exist. id=%v. err=%v", id, err)
-		return nil, nil, &ErrIssueContentHistoryNotExist{id}
+		return nil, nil, ErrIssueContentHistoryNotExist{id}
 	}
 
 	prevHistory = &ContentHistory{}