patch 8.2.3750: error messages are everywhere

Problem:    Error messages are everywhere.
Solution:   Move more error messages to errors.h and adjust the names.
diff --git a/src/popupwin.c b/src/popupwin.c
index 4b8b741..7641827 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -1877,7 +1877,7 @@
 	    buf = buflist_findnr(argvars[0].vval.v_number);
 	    if (buf == NULL)
 	    {
-		semsg(_(e_nobufnr), argvars[0].vval.v_number);
+		semsg(_(e_buffer_nr_does_not_exist), argvars[0].vval.v_number);
 		return NULL;
 	    }
 #ifdef FEAT_TERMINAL