[go-experimental] Fix error message when unmarshaling wrong enum value

[zippolyte]

The error message uses the unmarshaled struct, but it is empty since the unmarshal failed. This fixes it to have a proper error message.

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[zippolyte]

cc @antihax (2017/11) @bvwells (2017/12) @grokify (2018/07) @kemokemo (2018/09) @bkabrda (2019/07)

[bkabrda]

Nice catch, thanks!

[wing328]

CircleCI failure already fixed in the master.