catkin_prepare_release should validate metapackages

Specifically that they have valid `CMakeLists.txt`. A lot of people are going to run into this problem while releasing with bloom. It would be nice to have this tool check.

It could output:

```
cat <<EOF
Metapackage <metapackage_name> does not have a valid CMakeLists.txt, therefore it does not conform to REP-0127, see: http://ros.org/reps/rep-0127.html#metapackage

The CMakeLists.txt should look like this:
<CMakeLists.txt that was expected>
EOF
```

Instead of its normal command output, which would be safe for people doing something like this:

```
`catkin_prepare_release`
```