Throw on a missing file when loadJsonObject is asked to

Controller.loadJsonObject(path, true) returned {} for a missing file, so
loadPlugins' catch never ran and a plugin whose web build was expected but
missing was not reported (the second half of #1046; #1047 fixed the path).
With throwOnMissing set, every error now throws; without it only a missing
file is ignored, so a malformed JSON file throws again as before #1047.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BGUULoiHNbAegwMK4PSjHJ

packages/controller/src/Controller.ts

@@ -842,7 +842,7 @@ export default class Controller {
 		try {
 			manifest = JSON.parse(await fs.readFile(filePath, { encoding: "utf8" }));
 		} catch (err: any) {
-			if (throwOnMissing && err.code !== "ENOENT") {
+			if (throwOnMissing || err.code !== "ENOENT") {
 				throw err;
 			}
 		}