[web] fix uncaught exception when user cancels SIWE

Summary:
if a user cancels the request to sign the SIWE message from their wallet, we get an uncaught exception. wrap the call to signMessage with a try-catch to fix this bug

Depends on D12548

Test Plan: canceled SIWE message signing from my wallet and didn't get an uncaught exception

Reviewers: inka, ginsu

Reviewed By: ginsu

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12549