fix(eval): make error number of charidx() same as Vim

Author: zeertzjq

src/nvim/eval/funcs.c

@@ -908,7 +908,7 @@ static void f_charidx(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
     countcc = (int)tv_get_number(&argvars[2]);
   }
   if (countcc < 0 || countcc > 1) {
-    emsg(_(e_invarg));
+    semsg(_(e_using_number_as_bool_nr), countcc);
     return;
   }
 

src/nvim/testdir/test_functions.vim

@@ -1101,8 +1101,8 @@ func Test_charidx()
   call assert_fails('let x = charidx([], 1)', 'E474:')
   call assert_fails('let x = charidx("abc", [])', 'E474:')
   call assert_fails('let x = charidx("abc", 1, [])', 'E474:')
-  call assert_fails('let x = charidx("abc", 1, -1)', 'E474:')
-  call assert_fails('let x = charidx("abc", 1, 2)', 'E474:')
+  call assert_fails('let x = charidx("abc", 1, -1)', 'E1023:')
+  call assert_fails('let x = charidx("abc", 1, 2)', 'E1023:')
 endfunc
 
 func Test_count()