improve readme and error messages

[webern]

Issue #, if available:

Closes #334

Description of changes:

    readme: describe credentials behavio

    better error message for misconfiguration

    If AWS credentials are missing, i.e. there is no profile information
    in ~/.aws, and no other source of credentials, then coldsnap would
    provide this unhelpful error message:

    Failed to download snapshot: Failed to list snapshot blocks 'snapid'.

    We now print the underlying error as well, which is at least a little
    bit more helpful:

    Failed to download snapshot: Failed to list snapshot blocks 'snapid':
    dispatch failure: other: Invalid Configuration: Missing Region

Testing

Uploaded and Downloaded Snapshots. The error messages were like this before:

Failed to download snapshot: Failed to list snapshot blocks 'snap-0a5cdf61910a948d8': dispatch failure

Failed to upload snapshot: Failed to start snapshot: dispatch failure

And like this after

Failed to download snapshot: Failed to list snapshot blocks 'snap-0a5cdf61910a948d8': dispatch failure: other: Invalid Configuration: Missing Region

Failed to upload snapshot: Failed to start snapshot: dispatch failure: other: Invalid Configuration: Missing Region

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[webern]

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