Fix error messages changing with Node.js version

Updates to Node.js error reporting have caused tests to fail due to them
being hardcoded to expect a certain format that has since changed.  Fix
by making Node.js create the error message for us.

test/lib/data/ModPack.js

@@ -82,10 +82,16 @@ describe("lib/data/ModPack", function() {
 					() => ModPack.fromModPackString("AMalformedString"),
 					new Error("Malformed mod pack string: zlib inflate failed")
 				);
+				let badJsonMsg;
+				try {
+					JSON.parse("Not Json");
+				} catch (err) {
+					badJsonMsg = err.message;
+				}
 				assert.throws(
 					// eslint-disable-next-line node/no-sync
 					() => ModPack.fromModPackString(Buffer.from(zlib.deflateSync("Not Json")).toString("base64")),
-					new Error("Malformed mod pack string: Unexpected token N in JSON at position 0")
+					new Error(`Malformed mod pack string: ${badJsonMsg}`)
 				);
 				assert.throws(
 					// eslint-disable-next-line node/no-sync

test/lib/plugin_loader.js

@@ -45,9 +45,16 @@ describe("lib/plugin_loader", function() {
 			);
 		});
 		it("should reject on broken plugin", async function() {
+			let brokenMessage;
+			try {
+				// eslint-disable-next-line node/global-require
+				require(path.resolve(brokenPlugin, "info.js"));
+			} catch (err) {
+				brokenMessage = err.message;
+			}
 			await assert.rejects(
 				libPluginLoader.loadPluginInfos(new Map([["broken", path.resolve(brokenPlugin)]])),
-				{ message: "PluginError: Unexpected identifier" }
+				{ message: `PluginError: ${brokenMessage}` }
 			);
 		});
 		it("should reject on invalid plugin", async function() {