Handle errors in secret store constructors.

[jmcarp]

Fixes panic on bad aws configuration.

Before:

~/g/s/g/s/chamber 🤜 chamber list foo                                                         ✘ 130 fix-panic-bad-aws-session ✭ ◼
panic: AssumeRoleTokenProviderNotSetError: assume role with MFA enabled, but AssumeRoleTokenProvider session option not set.

goroutine 1 [running]:
github.com/aws/aws-sdk-go/aws/session.Must(...)
        /Users/joshuacarp/go/src/github.com/aws/aws-sdk-go/aws/session/session.go:281
github.com/segmentio/chamber/store.getSession(0xa, 0x16e7b92, 0x16)
        /Users/joshuacarp/go/src/github.com/segmentio/chamber/store/shared.go:21 +0x284
github.com/segmentio/chamber/store.NewSSMStore(0xa, 0x0)
        /Users/joshuacarp/go/src/github.com/segmentio/chamber/store/ssmstore.go:41 +0x2f
github.com/segmentio/chamber/cmd.getSecretStore(0x7ffeefbff435, 0x3)
        /Users/joshuacarp/go/src/github.com/segmentio/chamber/cmd/root.go:86 +0x8c
github.com/segmentio/chamber/cmd.list(0x1c48a20, 0xc000091710, 0x1, 0x1, 0x0, 0x0)
        /Users/joshuacarp/go/src/github.com/segmentio/chamber/cmd/list.go:36 +0xcc
github.com/spf13/cobra.(*Command).execute(0x1c48a20, 0xc0000916e0, 0x1, 0x1, 0x1c48a20, 0xc0000916e0)
        /Users/joshuacarp/go/src/github.com/spf13/cobra/command.go:762 +0x473
github.com/spf13/cobra.(*Command).ExecuteC(0x1c48ee0, 0xc0001c5f00, 0xc0001c5f88, 0x10071b0)
        /Users/joshuacarp/go/src/github.com/spf13/cobra/command.go:852 +0x2fd
github.com/segmentio/chamber/cmd.Execute(0x16df004, 0x3)
        /Users/joshuacarp/go/src/github.com/segmentio/chamber/cmd/root.go:54 +0x56
main.main()
        /Users/joshuacarp/go/src/github.com/segmentio/chamber/main.go:11 +0x39

After:

~/g/s/g/s/chamber 🤜 chamber list foo                                                         ✘ 2 fix-panic-bad-aws-session ✭ ◼
Error: Failed to get secret store: AssumeRoleTokenProviderNotSetError: assume role with MFA enabled, but AssumeRoleTokenProvider session option not set.

[jmcarp]

cc @nickatsegment @systemizer

[nickatsegment]

Unrelated, but I find non-main packages looking at the env to be very distasteful :P

[nickatsegment]

👍 getting rid of an os.Exit in a non-main package