Improve warning if model is missing a collision mesh

[srmainwaring]

The hydrodynamics plugin assumes that all links of a model have a valid collision mesh. If the mesh is missing the plugin fails to load correctly but does not supply a very helpful warning.

Suggested behaviour:

1. Either - ignore links with missing collision meshes
2. Or - emit a better error and report which link(s) are missing collision meshes

It may also be worth emitting warnings when the collision mesh contains a large number of vertices or near degenerate triangles which will give rise to poor numerical behaviour.