Add small check for bad shards

[hugopl]

If a shard has a git tag like v1.2.1, but the shards.yml in the repository says 1.2.0, shards build will always tell you to run shards update and you will end up blamming shards until notice it was a problem with the shard you were trying to use.

For sure this isn't a problem in shards itself, but such check is easy and a better message to the user would help a lot.

Not first time I ran into this "problem", a good error message would had saved some minutes.

[RX14]

Already solved by #341.