Make fault.Wrap nil tolerant

[spearson78]

I found it counter intuitive that fault.Wrap panicked when I passed a nil error.
I tried to upgrade my code by simply replacing

return x,err

with

return x,fault.Wrap(x,fctx.With(ctx))

However sometimes the err in my code is nil and fault.Wrap panicked.

For an error handling library it makes sense to me to not introduce panics into code.

I have these changes on my optional location branch (https://github.com/spearson78/fault/tree/floc). I can separate out the changes if needed.

[Southclaws]

Yeah this one is a leakage of some internal rules around errors, an opinionated take on when to wrap errors. We use some linting tools to prevent this but you're right, this shouldn't be a strongly opinionated library in this regard.

Happy to accept a PR for fault and the other sub-packages to resolve this!