patch 8.2.4740: when expand() fails there is no error message

Problem:    When expand() fails there is no error message.
Solution:   When 'verbose' is set give an error message.

runtime/doc/builtin.txt

@@ -2270,6 +2270,9 @@ expand({string} [, {nosuf} [, {list}]])				*expand()*
 		is not defined, an empty string is used.  Using "%:p" in a
 		buffer with no name, results in the current directory, with a
 		'/' added.
+		When 'verbose' is set then expanding '%', '#' and <> items
+		will result in an error message if the argument cannot be
+		expanded.
 
 		When {string} does not start with '%', '#' or '<', it is
 		expanded like a file name is expanded on the command line.

src/evalfunc.c

@@ -4065,7 +4065,6 @@ f_expand(typval_T *argvars, typval_T *rettv)
 {
     char_u	*s;
     int		len;
-    char	*errormsg;
     int		options = WILD_SILENT|WILD_USE_NL|WILD_LIST_NOTFOUND;
     expand_T	xpc;
     int		error = FALSE;
@@ -4096,9 +4095,15 @@ f_expand(typval_T *argvars, typval_T *rettv)
     s = tv_get_string(&argvars[0]);
     if (*s == '%' || *s == '#' || *s == '<')
     {
+	char	*errormsg = NULL;
+
+	if (p_verbose == 0)
 	    ++emsg_off;
 	result = eval_vars(s, s, &len, NULL, &errormsg, NULL);
+	if (p_verbose == 0)
 	    --emsg_off;
+	else if (errormsg != NULL)
+	    emsg(errormsg);
 	if (rettv->v_type == VAR_LIST)
 	{
 	    if (rettv_list_alloc(rettv) != FAIL && result != NULL)

src/testdir/test_expand.vim

@@ -128,13 +128,21 @@ func Test_source_sfile()
     :call assert_equal('edit <cword>', expandcmd("edit <cword>"))
     :call assert_equal('edit <cexpr>', expandcmd("edit <cexpr>"))
     :call assert_fails('autocmd User MyCmd echo "<sfile>"', 'E498:')
+    :
+    :call assert_equal('', expand('<script>'))
+    :verbose echo expand('<script>')
+    :call add(v:errors, v:errmsg)
+    :verbose echo expand('<sfile>')
+    :call add(v:errors, v:errmsg)
     :call writefile(v:errors, 'Xresult')
     :qall!
-
   [SCRIPT]
   call writefile(lines, 'Xscript')
   if RunVim([], [], '--clean -s Xscript')
-    call assert_equal([], readfile('Xresult'))
+    call assert_equal([
+          \ 'E1274: No script file name to substitute for "<script>"',
+          \ 'E498: no :source file name to substitute for "<sfile>"'],
+          \ readfile('Xresult'))
   endif
   call delete('Xscript')
   call delete('Xresult')

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4740,
 /**/
     4739,
 /**/