Use standard error interface for Claimer bot

[denalimarsh]

I would use the standard error interface instead of a custom one. It's designed to contain custom error types like we have here and ensures compatibility with other go code.

For line 62 there is an idiomatic way of checking an error type:

var errFailed ErrorFailed
if ok := errors.As(err, errFailed); ok {
    ...
}

It's more robust - if the err value is ever created in a non-main pkg then the %T won't be main.ErrorFailed.

The go blog is not bad for going through the idiomatic way to do errors in go 1.13+ https://blog.golang.org/go1.13-errors

Originally posted by @rhuairahrighairidh in #2