fn: agent/lb/runner error handling adjustments (#1214)

1) Early call validation and return due to cpu/mem impossible
to meet (eg. request cpu/mem larger than max-mem or max-cpu
on server) now emits HTTP Bad Request (400) instead of 503.
This case is most likely due to client/service configuration
and/or validation issue.
2) 'failed' metric is now removed. 'failed' versus 'errors'
were too confusing. 'errors' is now a catch all error case.
3) new 'canceled' counter for client side cancels.
4) 'server_busy' now covers more cases than it previously did.

api/models/error.go

@@ -161,6 +161,10 @@ var (
 		code:  http.StatusBadRequest,
 		error: fmt.Errorf("memory value is out of range. It should be between 0 and %d", RouteMaxMemory),
 	}
+	ErrCallResourceTooBig = err{
+		code:  http.StatusBadRequest,
+		error: fmt.Errorf("Requested CPU/Memory cannot be allocated"),
+	}
 	ErrCallNotFound = err{
 		code:  http.StatusNotFound,
 		error: errors.New("Call not found"),