Generalize error text (#1388)

* Generalize error text

Right now, each time there is an error while executing `substrate-relay`
it will be reported as:

    ERROR bridge Failed to start relay: <Actual cause of error>

This is the case even if the invoked command did not have anything to do
with starting a relayer. Thus this removes this text. Now something like
this would be written:

    ERROR bridge <Actual cause of error>

* Use substrate-relay prefix