fix: do not panic when the inventory object is missing

[gautierdelorme]

fixes #595

This patch adds back the user-friendly message when the inventory object is missing.

Before

$ kapply apply /foo/bar
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x4354dc2]

goroutine 162 [running]:
k8s.io/apimachinery/pkg/apis/meta/v1/unstructured.(*Unstructured).GetLabels(...)

After

$ kapply apply /foo/bar
Package uninitialized. Please run "init" command.
	
The package needs to be initialized to generate the template
which will store state for resource sets. This state is
necessary to perform functionality such as deleting an entire
package or automatically deleting omitted resources (pruning).

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[k8s-ci-robot]

Hi @gautierdelorme. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[karlkfi]

/ok-to-test

[karlkfi]

@seans3 are you ok with this? Seems ok with me, based on reasoning in #595

[gautierdelorme]

Just fixed the lint error in 38ab8f9 to use a switch statement instead of if.

[seans3]

/lgtm

[seans3]

Sean Sullivan are you ok with this? Seems ok with me, based on reasoning in #595

Yes, I'm OK with it. I've lgtm'ed. I'll let you approve if/when you're ok.

[karlkfi]

/approve

Thanks @gautierdelorme!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gautierdelorme, karlkfi

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