[x/tour] content: clarify return values and naked returns
[x/tour] content: clarify return values and naked returns

Fixes golang/go#12768

Change-Id: I18740d3fa002dcd8cef3261177a064f824a5a60a
Reviewed-on: https://go-review.googlesource.com/15080
Reviewed-by: Burcu Dogan <347c19a639ce752a783b4ea325d6539ab4b4a262@google.com>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>
X-Tour-Commit: ef51291e08d6b0383ff563ff33067c218982b38e

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website