missing gcloud creds results in panic - should tell user *why*

[eyberg]

right now it just blows up w/a panic - it should tell the user that it is missing credentials

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xe6e8a0]

goroutine 1 [running]:
cloud.google.com/go/storage.(*Client).Close(0x0, 0x0, 0x0)
        /home/eyberg/go/pkg/mod/cloud.google.com/go@v0.41.0/storage/storage.go:117 +0x20
github.com/nanovms/ops/lepton.(*GCPStorage).CopyToBucket(0x2451490, 0xc00052e200, 0xc0003d1350, 0x2f, 0x1826bc0, 0xc0000d0f40)
        /home/eyberg/go/src/github.com/nanovms/ops/lepton/gcpstore.go:24 +0x185
github.com/nanovms/ops/cmd.imageCreateCommandHandler(0xc0005f0c80, 0xc0000aa740, 0x0, 0x4)
        /home/eyberg/go/src/github.com/nanovms/ops/cmd/image.go:86 +0x619
github.com/spf13/cobra.(*Command).execute(0xc0005f0c80, 0xc0000aa680, 0x4, 0x4, 0xc0005f0c80, 0xc0000aa680)
        /home/eyberg/go/pkg/mod/github.com/spf13/cobra@v0.0.5/command.go:830 +0x2ae
github.com/spf13/cobra.(*Command).ExecuteC(0xc000101680, 0x1120d81, 0xc00014bf88, 0xc0000c6058)
        /home/eyberg/go/pkg/mod/github.com/spf13/cobra@v0.0.5/command.go:914 +0x2fc
github.com/spf13/cobra.(*Command).Execute(...)
        /home/eyberg/go/pkg/mod/github.com/spf13/cobra@v0.0.5/command.go:864
main.main()
        /home/eyberg/go/src/github.com/nanovms/ops/ops.go:8 +0x28