push: set generic error in push_negotiation cb

When a user returns `-1` in a `push_negotiation` callback, we set the
error to whatever's hanging out in the buffer, probably something about
a missing configuration entry.

Clear the error buffer before invoking the callback, so that if a user
does not set an error message in their callback that we can detect. If
there is no error but `-1` is returned, set a generic error message.

src/libgit2/push.c

@@ -444,10 +444,21 @@ static int do_push(git_push *push)
 	if ((error = calculate_work(push)) < 0)
 		goto on_error;
 
-	if (callbacks && callbacks->push_negotiation &&
-	    (error = callbacks->push_negotiation((const git_push_update **) push->updates.contents,
-					  push->updates.length, callbacks->payload)) < 0)
-	    goto on_error;
+	if (callbacks && callbacks->push_negotiation) {
+		git_error_clear();
+
+		error = callbacks->push_negotiation(
+			(const git_push_update **) push->updates.contents,
+			push->updates.length, callbacks->payload);
+
+		if (error < 0) {
+			git_error_set_after_callback_function(error,
+				"push_negotiation");
+			goto on_error;
+		}
+
+		error = 0;
+	}
 
 	if ((error = queue_objects(push)) < 0 ||
 	    (error = transport->push(transport, push)) < 0)