Error handling: Frontend to Bobtimus

[da-kami]

trafficstars

There is no error information passed back to the user. At the moment it can easily happen that a loan is not acceptable because the rate changed (there no time/price guarantees for offers at the moment).

For now I propose we at least introduce some error handling on the UI side so the user may "try again". In the long run it might be better to introduce more state on the lender side so we can tie requests to offers.

[da-kami]

this seems to be a duplicate of #200