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/popupwin.c b/src/popupwin.c
index cd343f8..8751bb4 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -61,7 +61,7 @@
 	    n = strtol((char *)s, (char **)&endp, 10);
 	if (endp != NULL && *skipwhite(endp) != NUL)
 	{
-	    semsg(_(e_invexpr2), val);
+	    semsg(_(e_invalid_expression_str), val);
 	    return 0;
 	}
     }