golint has gained a new check and we don't freeze the golint version so CI was
failing on unrelated PRs:

app/multitenant/consul_client.go:69:2: redundant if ...; err != nil check, just return error instead.
report/marshal.go:188:2: redundant if ...; err != nil check, just return error instead.

Fix those!

useful for codec and report structure experiments

rather than going via intermediate data structure

This avoids a runtime type lookup, so goes a little faster.
Also having less recursion makes it easier to interpret profiles.

This is more efficient, since the decoder can read field names in-place.
It also appears to be absolutely faster.