add dedicated error code for special case

ZSTD_compressSequencesAndLiterals() cannot produce an uncompressed block

lib/zstd_errors.h

@@ -76,6 +76,7 @@ typedef enum {
   ZSTD_error_tableLog_tooLarge       = 44,
   ZSTD_error_maxSymbolValue_tooLarge = 46,
   ZSTD_error_maxSymbolValue_tooSmall = 48,
+  ZSTD_error_cannotProduce_uncompressedBlock = 49,
   ZSTD_error_stabilityCondition_notRespected = 50,
   ZSTD_error_stage_wrong       = 60,
   ZSTD_error_init_missing      = 62,