Add kubectl error code to error messages

[sylr]

It happens that the controller logs an error with an empty message,e.g.:

{"level":"error","ts":"2020-11-09T13:55:26.336Z","logger":"controller","msg":"Reconciler error","reconcilerGroup":"kustomize.toolkit.fluxcd.io","reconcilerKind":"Kustomization","controller":"kustomization","name":"aks20-euw-infra-main-m
onitoring","namespace":"flux","error":"apply failed: "}

In that case the error message should mention that kubectl did not write anything on stderr and print the error code.