better error message when no creds are present

[tekumara]

Without any aws credentials, eks-iam-cache panics, eg:

$ kubectl get pods
open /Users/tekumara/.aws/eks-iam-cache-2ff47918aa2fff3b36865539d6ce22a1601bd1caa85b53248af63fa0cdf913dc.json: no such file or directory
panic: exit status 253

goroutine 1 [running]:
main.main()
	/Users/tekumara/go/pkg/mod/github.com/chlunde/[email protected]/main.go:89 +0x264
open /Users/tekumara/.aws/eks-iam-cache-2ff47918aa2fff3b36865539d6ce22a1601bd1caa85b53248af63fa0cdf913dc.json: no such file or directory
panic: exit status 253

Would be nice to show a more informative "no credentials" error instead.