storage: fix RocksDB error and go os error discrepancy

Because error returned from RocksDB is different from go's os error, we need to
bridge the gap by examining error message.

Release note: None

pkg/storage/engine/engine.go

@@ -288,6 +288,7 @@ type Engine interface {
 	// ReadFile reads the content from the file with the given filename int this RocksDB's env.
 	ReadFile(filename string) ([]byte, error)
 	// DeleteFile deletes the file with the given filename from this RocksDB's env.
+	// If the file with given filename doesn't exist, return os.ErrNotExist.
 	DeleteFile(filename string) error
 }
 

pkg/storage/engine/rocksdb.go

@@ -2652,6 +2652,7 @@ func (r *RocksDB) ReadFile(filename string) ([]byte, error) {
 }
 
 // DeleteFile deletes the file with the given filename from this RocksDB's env.
+// If the file with given filename doesn't exist, return os.ErrNotExist.
 func (r *RocksDB) DeleteFile(filename string) error {
 	if err := statusToError(C.DBEnvDeleteFile(r.rdb, goToCSlice([]byte(filename)))); err != nil {
 		return notFoundErrOrDefault(err)
@@ -2706,7 +2707,7 @@ func mvccScanDecodeKeyValue(repr []byte) (key MVCCKey, value []byte, orepr []byt
 }
 
 func notFoundErrOrDefault(err error) error {
-	if strings.Contains(err.Error(), "No such file or directory") {
+	if strings.Contains(err.Error(), "No such file or directory") || strings.Contains(err.Error(), "File not found") {
 		return os.ErrNotExist
 	}
 	return err