Fail if the mesh isn't valid

This changed is based on the 4th revision of cleaver
(MD5 cdbea5baead566c6d284cfc7461fd682) that prevents it from exiting upon
error (it returns instead). It means that the CLI needs to check for the
validity of the mesh generated.
Merge branch 'CheckTetMeshValidy' into next

* CheckTetMeshValidy:
  Fail if the mesh isn't valid