Throw an error if an incomplete profile is encountered

[jamesls]

This is along the same lines as #133
If a user has a profile configured but it is missing keys, we fall back to other credential mechanisms. For example, if I have a config file of:

[profile foo]
aws_access_key_id=foo
aws_secret_access_MISPELLED=bar

And I run aws service op --profile foo, then we'll silently fall back to other credentials provider. If I had a valid ~/.boto config, then we'd silently use the boto config, which might have surprising results.

I think we should update the CLI in the same spirit as #133. If you specify a --profile, it has to exist in its entirety, otherwise an error is thrown. This means we should at least require access/secret key entries in the profile.