Respond to unhandled requests

Give an error response back to unhandled requests, this fixes requests
hanging forever when the target does not support the given request.

packages/lib/link/link.js

@@ -39,6 +39,18 @@ class Link {
 					if (err.errors) {
 						console.error(err.errors);
 					}
+
+					// Send back an error response if this was a request.
+					if (
+						typeof payload.type === "string"
+						&& payload.type.endsWith("_request")
+						&& Number.isInteger(payload.seq)
+					) {
+						this.connector.send(
+							`${payload.type.slice(0, -8)}_response`,
+							{ seq: payload.seq, error: err.message }
+						);
+					}
 				} else {
 					this.connector.emit("error", err);
 				}

test/lib/link/link.js

@@ -35,6 +35,16 @@ describe("lib/link/link", function() {
 			await assert.rejects(result, new Error("Handler error"));
 		});
 
+		it("should give an error response back on an unhandled request", function() {
+			testConnector.sentMessages = [];
+			testConnector.emit("message", { seq: 1, type: "unhandled_request", data: {}});
+			assert.deepEqual(testConnector.sentMessages, [{
+				type: "unhandled_response",
+				seq: testConnector._seq - 1,
+				data: { seq: 1, error: "No validator for unhandled_request on source-target" } },
+			])
+		});
+
 		describe(".setValidator()", function() {
 			it("should set the validator", function() {
 				testLink.setValidator("test", () => true);