operator describe: don't panic when specified package name is not found

[joelanford]

What happens:

$ kubectl operator describe foo        
panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
github.com/operator-framework/kubectl-operator/internal/cmd.newOperatorDescribeCmd.func1(0x14000160c80?, {0x1400011a7e0?, 0x1?, 0x1?})
        kubectl-operator/internal/cmd/operator_describe.go:51 +0xba4
github.com/spf13/cobra.(*Command).execute(0x14000160c80, {0x1400011a7a0, 0x1, 0x1})
        /Users/joe/go/pkg/mod/github.com/spf13/cobra@v1.4.0/command.go:860 +0x4ac
github.com/spf13/cobra.(*Command).ExecuteC(0x14000142c80)
        /Users/joe/go/pkg/mod/github.com/spf13/cobra@v1.4.0/command.go:974 +0x354
github.com/spf13/cobra.(*Command).Execute(...)
        /Users/joe/go/pkg/mod/github.com/spf13/cobra@v1.4.0/command.go:902
github.com/operator-framework/kubectl-operator/internal/cmd.Execute()
        kubectl-operator/internal/cmd/root.go:16 +0x20
main.main()
        kubectl-operator/main.go:8 +0x1c

What I expect to happen (and what happens with this PR):

$ kubectl operator describe foo
packagemanifests.packages.operators.coreos.com "foo" not found

Follows up on #78

Signed-off-by: Joe Lanford joe.lanford@gmail.com

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