Fix "Invalid prefix operation" errors (#4577)

`--(-a);` returned with a postfix error before this patch.
The changes introduce a new warning for prefix errors, and
returns with the corresponding one based on current opcode.

JerryScript-DCO-1.0-Signed-off-by: Bela Toth tbela@inf.u-szeged.hu

jerry-core/parser/js/js-parser-util.c

@@ -1110,6 +1110,10 @@ parser_error_to_string (parser_error_t error) /**< error code */
     {
       return "Invalid left-hand side expression in postfix operation";
     }
+    case PARSER_ERR_INVALID_LHS_PREFIX_OP:
+    {
+      return "Invalid left-hand side expression in prefix operation";
+    }
     case PARSER_ERR_INVALID_LHS_FOR_LOOP:
     {
       return "Invalid left-hand-side in for-loop";