Audit usages of `assert` outside of tests

[mkurapov]

There are a handful of usages of assert outside of tests. An example in the mock account provider: https://github.com/interledger/rafiki/blob/main/packages/mock-account-provider/app/lib/accounts.server.ts#L49-L50

We should convert these to throw applicable Errors instead, and reserve assert usages for tests only.