Add messaging on how to enable sigv4 for s3

[kyleknap]

Now the error will now look like this:

$ aws s3 cp s3://mybucketfoo/foo foo
download failed: s3://mybucketfoo/foo to ./foo A client error (InvalidArgument) occurred when calling the GetObject operation: Requests specifying Server Side Encryption with AWS KMS managed keys require AWS Signature Version 4. You can enable AWS Signature Version 4 by running the command: 
aws configure set s3.signature_version s3v4

Was brought up in discussion of this PR: #1623

cc @jamesls @mtdowling @rayluo @JordonPhillips

[mtdowling]

🚢

[JordonPhillips]