validate: Make missing <releases> more obvious

[jtojnar]

Previously, when <release> was not contained in <releases>, the following confusing error message was printed:

• tag-missing           : <release> required

Now, the validation will print

• tag-missing           : <releases> required

when is missing altogether and

• tag-missing           : <release> required in <releases>

when the <releases> tag does not contain any <release>.

Fixes: #375

[jtojnar]

This slightly improves that particular confusing warning but it is still far from ideal since it is too weak model at the moment. Ideally, we would use something like XSD schema for doing the boring validity check and only write code for checking things that cannot be expressed in schemas.