When region is `nil` in `signing_key/3` resulting error isn't very clear

[tudborg]

In signing_key/3, if :region isn't set (or is set to nil) in config:

ex_aws/lib/ex_aws/auth/signatures.ex

Lines 12 to 18 in a0e9c61

	defp signing_key(service, datetime, config) do
	["AWS4", config[:secret_access_key]]
	|> hmac_sha256(date(datetime))
	|> hmac_sha256(config[:region])
	|> hmac_sha256(service)
	|> hmac_sha256("aws4_request")
	end

the resulting error is something like

** (ErlangError) Erlang error: {:badarg, {~c"mac.c", 193}, ~c"Bad text"}
    (crypto 5.7) crypto.erl:1218: :crypto.mac(:hmac, :sha256, <<50, 232, ...>>, nil)
    (ex_aws 2.6.0) lib/ex_aws/auth/signatures.ex:15: ExAws.Auth.Signatures.signing_key/3
    (ex_aws 2.6.0) lib/ex_aws/auth/signatures.ex:7: ExAws.Auth.Signatures.generate_signature_v4/4
    (ex_aws 2.6.0) lib/ex_aws/auth.ex:136: ExAws.Auth.auth_header/7
    (ex_aws 2.6.0) lib/ex_aws/auth.ex:55: ExAws.Auth.headers/6
    (ex_aws 2.6.0) lib/ex_aws/request.ex:27: ExAws.Request.request_and_retry/7
    (ex_aws 2.6.0) lib/ex_aws/operation/s3.ex:40: ExAws.Operation.ExAws.Operation.S3.perform/2
    (ex_aws_s3 2.5.8) lib/ex_aws/s3/upload.ex:58: ExAws.S3.Upload.initialize/2

which isn't super helpful.

The underlying problem is that the data argument passed to crypto.mac/4 is nil (from config[:region])
In my case, I had region: System.get_env("AWS_REGION") which wasn't set in the environment.
Which I should obviously just make sure is set.

Would you accept a patch that throws a better error?

Maybe even just

    |> hmac_sha256(Map.fetch!(config, :region))