Report error when kubectl unavailable during context check
Report error when kubectl unavailable during context check

During checks of production and staging Kubernetes contexts, report an
error when the `kubectl` executable is unavailable.

These checks currently raise an exception when `kubectl` is unavailable:

```
ERROR: Command 'kubectl config view' returned non-zero exit status 127
```

We can instead display errors to the user:

```
staging context not found
production context not found
```

izakp/hokusai