patch 8.2.0493: Vim9: some error messages not tested

Problem:    Vim9: some error messages not tested.
Solution:   Add more tests.  Fix uncovered bugs.
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 2726e38..ce1ca76 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -766,7 +766,7 @@
 
 /*
  * Generate an ISN_PUSHPARTIAL instruction with partial "part".
- * Consumes "name".
+ * Consumes "part".
  */
     static int
 generate_PUSHPARTIAL(cctx_T *cctx, partial_T *part)
@@ -3808,9 +3808,6 @@
 	    case VAR_BOOL:
 		generate_PUSHBOOL(cctx, VVAL_FALSE);
 		break;
-	    case VAR_SPECIAL:
-		generate_PUSHSPEC(cctx, VVAL_NONE);
-		break;
 	    case VAR_FLOAT:
 #ifdef FEAT_FLOAT
 		generate_PUSHF(cctx, 0.0);
@@ -3843,6 +3840,7 @@
 	    case VAR_NUMBER:
 	    case VAR_UNKNOWN:
 	    case VAR_VOID:
+	    case VAR_SPECIAL:  // cannot happen
 		generate_PUSHNR(cctx, 0);
 		break;
 	}