MAC: Avoid always-nil error

[oncilla]

The spath.Hop.CalcMac unnecessarily returns an error. It is always nil.

This PR removes the error from the returned values and reduces the
error handling branches in the code.

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This change is 

[scrye]

Reviewed 9 of 9 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @oncilla)

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go/lib/spath/hop.go, line 155 at r1 (raw file):

	mac.Reset()
	// We can safely ignore the return values: https://godoc.org/hash#Hash
	mac.Write(all)

I would add a panic if the Write returns an error; it would catch faulty implementations of hash.Hash.

[oncilla]

Reviewable status: 7 of 9 files reviewed, 1 unresolved discussion (waiting on @scrye)

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go/lib/spath/hop.go, line 155 at r1 (raw file):

Previously, scrye (Sergiu Costea) wrote…

I would add a panic if the Write returns an error; it would catch faulty implementations of hash.Hash.

Done.

[scrye]

Reviewed 2 of 3 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved

[scrye]

Reviewed 1 of 2 files at r3.
Reviewable status: complete! all files reviewed, all discussions resolved