Properly error out when input file doesn't exist

[blag]

Description of the change

This fixes a bug in the kubeseal CLI where it silently and errorneously does nothing and returns a 0 (eg: success) exit code when a specified --secret-file does not exist (or is unreadable).

$ kubeseal \
    --kubeconfig ${KUBECONFIG:-~/.kube/config} \
    --controller-namespace=sealed-secrets \
    --controller-name=sealed-secrets-controller \
    --secret-file doesntexist.yaml; echo $?
0

It looks like there was a simple oversight in #1030, and as noted in that PR:

As with all refactors, bugs could be introduced.

I just tweaked the exit logic to output an error message and exit with nonzero status, which I think is much more reasonable behavior as it is both less surprising and also follows what other POSIX-y CLIs do when they are pointed to files that don't exist.

Benefits

Instead of silently doing nothing and erroneously returning a success status/exit code, this will now clearly indicate that the problem occurred and programmatically indicate a non-successful result. I'm not super happy with the error message (I think it could be written better), but this is my first PR for a Go project, so I'm not sure what the culture of error messages is in the Go community (constructive guidance will be welcome and appreciated!).

Possible drawbacks

Anybody who has been relying on the previous -- non-standard -- behavior will be impacted by this change. But, also as noted in #1030:

The currently exposed kubeseal interface must be considered unstable, it will change going forward.

Applicable issues

• fixes #

Additional information

This is my first ever foray into Golang, so I might need more hand-holding than your average contributor, especially around how to test this. TIA. 😄

Edit: Added example.