saml2aws exec does not work if not logged in

[hoegertn]

$ saml2aws exec bash
Adding osx helper
NoCredentialProviders: no valid providers in chain. Deprecated.
	For verbose messaging see aws.Config.CredentialsChainVerboseErrors
error validating token
github.com/versent/saml2aws/cmd/saml2aws/commands.Exec
	/Users/markw/Code/go/src/github.com/versent/saml2aws/cmd/saml2aws/commands/exec.go:26
main.main
	/Users/markw/Code/go/src/github.com/versent/saml2aws/cmd/saml2aws/main.go:75
runtime.main
	/usr/local/Cellar/go/1.8.1/libexec/src/runtime/proc.go:185
runtime.goexit
	/usr/local/Cellar/go/1.8.1/libexec/src/runtime/asm_amd64.s:2197

It happens in the checkToken function. It should handle more AWS errors by going to login.