Update message for error during push

When attempting to update a reference on a remote during push, and the
reference on the remote refers to a commit that does not exist locally,
then we should report a more clear error message.

src/push.c

diff --git a/src/push.c b/src/push.c
index be5ec1c..8a50fb4 100644
--- a/src/push.c
+++ b/src/push.c
@@ -360,7 +360,8 @@
 				continue;
 
 			if (!git_odb_exists(push->repo->_odb, &spec->roid)) {
-				giterr_set(GITERR_REFERENCE, "Cannot push missing reference");
+				giterr_set(GITERR_REFERENCE, 
+					"Cannot push because a reference that you are trying to update on the remote contains commits that are not present locally.");
 				error = GIT_ENONFASTFORWARD;
 				goto on_error;
 			}