fix(inccommand): block errors when parsing command line again (#24374)

Revert the change to ex_getln.c from a741c7fd04

src/nvim/ex_getln.c

@@ -261,6 +261,7 @@ static bool do_incsearch_highlighting(int firstc, int *search_delim, incsearch_s
     return false;
   }
 
+  emsg_off++;
   exarg_T ea = {
     .line1 = 1,
     .line2 = 1,
@@ -368,6 +369,7 @@ static bool do_incsearch_highlighting(int firstc, int *search_delim, incsearch_s
   curwin->w_cursor = save_cursor;
   retval = true;
 theend:
+  emsg_off--;
   return retval;
 }
 
@@ -2427,9 +2429,12 @@ static bool cmdpreview_may_show(CommandLineState *s)
   int cmdpreview_type = 0;
   char *cmdline = xstrdup(ccline.cmdbuff);
   const char *errormsg = NULL;
+  emsg_off++;  // Block errors when parsing the command line, and don't update v:errmsg
   if (!parse_cmdline(cmdline, &ea, &cmdinfo, &errormsg)) {
+    emsg_off--;
     goto end;
   }
+  emsg_off--;
 
   // Check if command is previewable, if not, don't attempt to show preview
   if (!(ea.argt & EX_PREVIEW)) {