Test rhcos.upgrade.from-ocp-rhcos panics on 4.11

[jlebon]

15:11:16  --- FAIL: rhcos.upgrade.from-ocp-rhcos (2.62s)
15:11:16  panic: runtime error: invalid memory address or nil pointer dereference [recovered]
15:11:16  	panic: runtime error: invalid memory address or nil pointer dereference
15:11:16  [signal SIGSEGV: segmentation violation code=0x1 addr=0x78 pc=0x172b547]
15:11:16  
15:11:16  goroutine 291 [running]:
15:11:16  github.com/coreos/mantle/harness.tRunner.func1()
15:11:16  	/root/containerbuild/mantle/harness/harness.go:491 +0x23e
15:11:16  panic({0x1b01600, 0x30e9450})
15:11:16  	/usr/lib/golang/src/runtime/panic.go:838 +0x207
15:11:16  github.com/coreos/mantle/kola/tests/rhcos.downloadLatestReleasedRHCOS({0x1f6dce4, 0x4})
15:11:16  	/root/containerbuild/mantle/kola/tests/rhcos/upgrade.go:377 +0x987
15:11:16  github.com/coreos/mantle/kola/tests/rhcos.rhcosUpgradeFromOcpRhcos({0xc00048de00, {0x229db10, 0xc0006aaf90}, {0x0, 0x0, 0x0}, 0x1, 0x0})
15:11:16  	/root/containerbuild/mantle/kola/tests/rhcos/upgrade.go:213 +0x1c5
15:11:16  github.com/coreos/mantle/kola.runTest(0xc00048de00, 0xc0000012c0, {0x7ffe6536e352?, 0xb?}, {0x229b558, 0xc00096aa20})
15:11:16  	/root/containerbuild/mantle/kola/harness.go:1500 +0x954
15:11:16  github.com/coreos/mantle/kola.runProvidedTests.func1(0x2299c90?)
15:11:16  	/root/containerbuild/mantle/kola/harness.go:674 +0x68
15:11:16  github.com/coreos/mantle/harness.tRunner(0xc00048de00, 0xc0009e2090)
15:11:16  	/root/containerbuild/mantle/harness/harness.go:533 +0x10a
15:11:16  created by github.com/coreos/mantle/harness.(*H).RunTimeout
15:11:16  	/root/containerbuild/mantle/harness/harness.go:579 +0x42d

I think this is a combination of missing error-handling (fixed by #3269), not handling all the cases in the graph (fixed by #3266), and needing to adapt to the new bucket layout (fix pending).