make it easier to debug aws-env configuration issues

[ketzacoatl]

When configuring aws-env for the first time, it's often difficult to get it 100% the first time, and debugging what is actually wrong with the setup is excruciatingly difficult to walk a new though (at least for me). I would greatly appreciate --verbose logging or similar improvements that make it easier to identify where there is a mis-configuration (and what needs to be done to get the tool properly configured). cc @borsboom

[neilmayhew]

It's possible to use bash -x to run it, which is a bit crude, but better than nothing.