render plain text file if the LFS object doesn't exist (#31812)

We had an issue where a repo was using LFS to store a file, but the user
did not push the file. When trying to view the file, Gitea returned a
500 HTTP status code referencing `ErrLFSObjectNotExist`. It appears the
intent was the render this file as plain text, but the conditional was
flipped. I've also added a test to verify that the file is rendered as
plain text.

routers/web/repo/view.go

@@ -234,14 +234,12 @@ func getFileReader(ctx gocontext.Context, repoID int64, blob *git.Blob) ([]byte,
 	}
 
 	meta, err := git_model.GetLFSMetaObjectByOid(ctx, repoID, pointer.Oid)
-	if err != nil && err != git_model.ErrLFSObjectNotExist { // fallback to plain file
+	if err != nil { // fallback to plain file
+		log.Warn("Unable to access LFS pointer %s in repo %d: %v", pointer.Oid, repoID, err)
 		return buf, dataRc, &fileInfo{isTextFile, false, blob.Size(), nil, st}, nil
 	}
 
 	dataRc.Close()
-	if err != nil {
-		return nil, nil, nil, err
-	}
 
 	dataRc, err = lfs.ReadMetaObject(pointer)
 	if err != nil {