[coordinator] only log stack traces on panic (#1480)
[coordinator] only log stack traces on panic (#1480)

It's often clear where errors are coming from. When stack traces are
enabled, if a coordinator is being used as a prom backend and there's an
error with the M3DB cluster then every single prom write causes a stack
trace and the logs quickly fill up, making debugging difficult.
m3db/m3db,m3db/m3db,m3db/m3db,m3db/m3db