Review error and exception handling

[holisticode]

Currently we have a working "happy-day" code: Everything related to Swap works for the normal operation scenario.

We need to review error cases and unexpected flow situations and handle them.

[mortelli]

Making a note of @zelig's comment, on swarm/swap/types.go:53:

this is a mistake i often made and resented.
Try to avoid idle code, and "we may need it in future" complexities.

Originally posted in #1554

[Eknir]

Related: #1635