files: show newlines in filenames as ^J also in error messages

When a given file is, for example, unreadable or unwritable, the
error message should use ^J instead of ^@ in its name.

src/files.c

@@ -433,6 +433,9 @@ bool open_buffer(const char *filename, bool undoable)
 
     assert(filename != NULL);
 
+    /* Display newlines in filenames as ^J. */
+    as_an_at = FALSE;
+
 #ifndef DISABLE_OPERATINGDIR
     if (check_operating_dir(filename, FALSE)) {
 	statusline(ALERT, _("Can't insert file from outside of %s"),
@@ -1058,7 +1061,7 @@ void do_insertfile(void)
     bool execute = FALSE, right_side_up = FALSE, single_line = FALSE;
 #endif
 
-    /* Display embedded newlines as ^J. */
+    /* Display newlines in filenames as ^J. */
     as_an_at = FALSE;
 
     while (TRUE) {
@@ -2181,7 +2184,7 @@ int do_writeout(bool exiting)
     static bool did_credits = FALSE;
 #endif
 
-    /* Display embedded newlines as ^J. */
+    /* Display newlines in filenames as ^J. */
     as_an_at = FALSE;
 
     if (exiting && ISSET(TEMP_FILE) && openfile->filename[0] != '\0') {