patch 8.2.4003: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more error messages to errors.h.
diff --git a/src/sign.c b/src/sign.c
index 290d414..42592a4 100644
--- a/src/sign.c
+++ b/src/sign.c
@@ -933,7 +933,7 @@
 	    if (next_sign_typenr == start)
 	    {
 		vim_free(sp);
-		emsg(_("E612: Too many signs defined"));
+		emsg(_(e_too_many_signs_defined));
 		return NULL;
 	    }
 	    lp = first_sign;  // start all over