Warn on SSH connection for incorrect configuration (#19317) (#19437)

Backport #19317

- Warn on SSH connection for incorrect configuration
- When `setting.RepoRootPath` cannot be found(most likely due to
incorrect configuration) show "Gitea: Incorrect configuration" on the
client-side to help easier with debugging the problem.

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

cmd/serv.go

@@ -297,6 +297,15 @@ func runServ(c *cli.Context) error {
 		gitcmd = exec.CommandContext(ctx, verb, repoPath)
 	}
 
+	// Check if setting.RepoRootPath exists. It could be the case that it doesn't exist, this can happen when
+	// `[repository]` `ROOT` is a relative path and $GITEA_WORK_DIR isn't passed to the SSH connection.
+	if _, err := os.Stat(setting.RepoRootPath); err != nil {
+		if os.IsNotExist(err) {
+			return fail("Incorrect configuration.",
+				"Directory `[repository]` `ROOT` was not found, please check if $GITEA_WORK_DIR is passed to the SSH connection or make `[repository]` `ROOT` an absolute value.")
+		}
+	}
+
 	gitcmd.Dir = setting.RepoRootPath
 	gitcmd.Stdout = os.Stdout
 	gitcmd.Stdin = os.Stdin