Update invalid state error message on reboot API

This change adds more detail to the error message
when a request to soft reboot an instance in an
invalid state fails, indicating that specifically
soft reboot is the request that failed, when
a hard reboot might still succeed.

Closes-Bug: 1365683

Change-Id: I4b12aa261d1c4fa05bacd5311793ed6e0478e59b
Merge "Update invalid state error message on reboot API"