patch 8.2.3987: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more error messages to errors.h.
diff --git a/src/term.c b/src/term.c
index b3df1b3..85f6229 100644
--- a/src/term.c
+++ b/src/term.c
@@ -2222,13 +2222,13 @@
 
 	if (i < 0)
 # ifdef TGETENT_ZERO_ERR
-	    return _("E557: Cannot open termcap file");
+	    return _(e_cannot_open_termcap_file);
 	if (i == 0)
 # endif
 #ifdef TERMINFO
-	    return _("E558: Terminal entry not found in terminfo");
+	    return _(e_terminal_entry_not_found_in_terminfo);
 #else
-	    return _("E559: Terminal entry not found in termcap");
+	    return _(e_terminal_entry_not_found_in_termcap);
 #endif
     }
     return NULL;