User facing messages for AGit errors (#33012)

Adds user facing messages to errors when submitting agit pull request
Tries to highlight the returned error more and fixes agit suggestion to
create PR on first submission.

Closes: https://github.com/go-gitea/gitea/issues/32965

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/agit/agit.go

@@ -153,7 +153,7 @@ func ProcReceive(ctx context.Context, repo *repo_model.Repository, gitRepo *git.
 				OriginalRef:       opts.RefFullNames[i],
 				OldOID:            objectFormat.EmptyObjectID().String(),
 				NewOID:            opts.NewCommitIDs[i],
-				IsCreatePR:        true,
+				IsCreatePR:        false, // AGit always creates a pull request so there is no point in prompting user to create one
 				URL:               fmt.Sprintf("%s/pulls/%d", repo.HTMLURL(), pr.Index),
 				ShouldShowMessage: setting.Git.PullRequestPushMessage && repo.AllowsPulls(ctx),
 				HeadBranch:        headBranch,