Sign Message should handle non-string payloads

[piyalbasu]

In Sign Message, if a user passes uint32array, the app throws an error because it's unable to parse it. This view should be able to sanitize non-string inputs and give the user a useful message to let them know what went wrong.

Related Sentry alert: https://stellarorg.sentry.io/issues/7164546048/?referrer=slack&notification_uuid=04bbeb8a-c6a6-4348-86ed-ac87fd557d53&alert_rule_id=15034055&alert_type=issue