Add reporting mechanisms for why agents aren't moving.

[andriyDev]

We need to know why agent's aren't doing anything! Some obvious errors to report:

• Agent is not on the nav mesh.
• Target is not on the nav mesh.
• Found no path to the target (along with stats about the search).

[andriyDev]

This is pretty much resolved with AgentCurrentState and AgentState. We still don't have stats for path searches, but that depends on https://github.com/andriyDev/landmass/issues/8.