patch 8.2.3069: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move some error messages to errors.h.  Use clearer names.
diff --git a/src/buffer.c b/src/buffer.c
index d038e3c..59f51aa 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -2407,7 +2407,7 @@
     if (buf == NULL)
     {
 	if ((options & GETF_ALT) && n == 0)
-	    emsg(_(e_noalt));
+	    emsg(_(e_no_alternate_file));
 	else
 	    semsg(_("E92: Buffer %d not found"), n);
 	return FAIL;
@@ -3516,7 +3516,7 @@
     if (buflist_name_nr(0, &fname, &dummy) == FAIL)
     {
 	if (errmsg)
-	    emsg(_(e_noalt));
+	    emsg(_(e_no_alternate_file));
 	return NULL;
     }
     return fname;