Handle the error of a missing blob object fix #19530 (#19552)

* Handle the error of a missing blob object

* Show error in logs

* as per @zeripath

* Add missing error check

* Add missing error check

* Update compare.go

* Use formal code

* Update compare.go

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

routers/web/repo/compare.go

@@ -143,6 +143,11 @@ func setCsvCompareContext(ctx *context.Context) {
 		if err == errTooLarge {
 			return CsvDiffResult{nil, err.Error()}
 		}
+		if err != nil {
+			log.Error("CreateCsvDiff error whilst creating baseReader from file %s in commit %s in %s: %v", diffFile.Name, baseCommit.ID.String(), ctx.Repo.Repository.Name, err)
+			return CsvDiffResult{nil, "unable to load file from base commit"}
+		}
+
 		headReader, headBlobCloser, err := csvReaderFromCommit(&markup.RenderContext{Ctx: ctx, Filename: diffFile.Name}, headCommit)
 		if headBlobCloser != nil {
 			defer headBlobCloser.Close()
@@ -150,13 +155,17 @@ func setCsvCompareContext(ctx *context.Context) {
 		if err == errTooLarge {
 			return CsvDiffResult{nil, err.Error()}
 		}
+		if err != nil {
+			log.Error("CreateCsvDiff error whilst creating headReader from file %s in commit %s in %s: %v", diffFile.Name, headCommit.ID.String(), ctx.Repo.Repository.Name, err)
+			return CsvDiffResult{nil, "unable to load file from head commit"}
+		}
 
 		sections, err := gitdiff.CreateCsvDiff(diffFile, baseReader, headReader)
 		if err != nil {
 			errMessage, err := csv_module.FormatError(err, ctx.Locale)
 			if err != nil {
-				log.Error("RenderCsvDiff failed: %v", err)
+				log.Error("CreateCsvDiff FormatError failed: %v", err)
-				return CsvDiffResult{nil, ""}
+				return CsvDiffResult{nil, "unknown csv diff error"}
 			}
 			return CsvDiffResult{nil, errMessage}
 		}