respect json Accept header in API error responses

backend/src/utils/api.ts

@@ -0,0 +1,9 @@
+import { Request, Response } from 'express';
+
+export function handleError(req: Request, res: Response, statusCode: number, errorMessage: string | unknown): void {
+  if (req.accepts('json')) {
+    res.status(statusCode).json({ error: errorMessage });
+  } else {
+    res.status(statusCode).send(errorMessage);
+  }
+}