errors can be misleading when environments are specified incorrectly

[William Reade]

If you have (say) an orchestra environment specified, but missing a required field, the error raised during schema validation will complain about an invalid ec2 environment (because ec2 happens to be the first one specified in ensemble.environment.config, and is the first one to raise an error, and the orchestra schema validation's error is not sufficiently interesting to override the ec2 error).

This is bad because, if someone has clearly specified an orchestra environment, they will be notably unimpressed to see an error complaining that their ec2 environment has not specified a control-bucket.