issue 28: fix wrong location of some errors

[matdurand]

Fixes part of #28
This change introduce the following in the flatten method:

• non-fault error results in a step without a location
• all context (ftag, fmsg) attached to a wrap call will share the same location
• fault.New will use its own location
• empty .Wrap(err) function results in a Step with a location but without message

[Southclaws]

Brilliant thanks! It's great to see more location info in those error chains, but not as noisy as a full stack trace.