[FIX] Headers already sent error when user data download is disabled

[sampaiodiego]

Proposed changes (including videos or screenshots)

When using the export message tool when trying to download the file using the link sent via email if the feature "Export User Data" is disabled an error was being thrown causing the request to halt.

This is the error shown in the logs:

=== UnHandledPromiseRejection ===
Error [ERR_HTTP_HEADERS_SENT] [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at ServerResponse.setHeader (_http_outgoing.js:530:11)
    at ServerResponse.res.setHeader (/app/bundle/programs/server/npm/node_modules/meteor/simple_json-routes/node_modules/connect/lib/patch.js:134:22)
    at app/user-data-download/server/exportDownload.js:14:7
    at /app/bundle/programs/server/npm/node_modules/meteor/promise/node_modules/meteor-promise/fiber_pool.js:43:40 {
  code: 'ERR_HTTP_HEADERS_SENT'
}
---------------------------------
Errors like this can cause oplog processing errors.
Setting EXIT_UNHANDLEDPROMISEREJECTION will cause the process to exit allowing your service to automatically restart the process
Future node.js versions will automatically exit the process
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