Remove double RefreshState on fleet startup. Various other logging and error handling cleanup.

as best as I can tell, this RefreshState() in Agent startup is redundant with the one performed just beforehand during Server creation. I noticed this from the dupe log messages:

```
I0403 18:41:56.616462 11868 fleet.go:135] Continuing without config file
I0403 18:41:56.616543 11868 machine.go:20] Created Machine with static state {  map[] 0.1.4+git}
I0403 18:41:56.616589 11868 state.go:85] Attempting to retrieve IP route info from netlink
I0403 18:41:56.616964 11868 state.go:103] Found default route with interface wlp2s0
I0403 18:41:56.620516 11868 agent.go:95] Initializing Agent
I0403 18:41:56.620557 11868 state.go:85] Attempting to retrieve IP route info from netlink
I0403 18:41:56.621083 11868 state.go:103] Found default route with interface wlp2s0
```
 
after the change:
```
I0403 18:42:47.295546 11983 fleet.go:144] No provided or default config file found - proceeding without
I0403 18:42:47.295620 11983 fleet.go:63] Creating Server
I0403 18:42:47.295630 11983 machine.go:20] Created Machine with static state {  map[] 0.1.4+git}
I0403 18:42:47.295705 11983 state.go:85] Attempting to retrieve IP route info from netlink
I0403 18:42:47.296046 11983 state.go:103] Found default route with interface wlp2s0
I0403 18:42:47.297553 11983 agent.go:95] Initializing Agent
```

I also manually verified that the MachineState is identical between the two, but couldn't think of how to wire up a good integration test for this without some significant refactoring.