Improving error messages in case of double key in config

[iitzco-asapp]

Is your feature request related to a problem? Please describe.
When certain configs have a duplicate key in the config, then flambe fails with:

ruamel.yaml.constructor.ConstructorError: could not determine a constructor for the tag xxxx

I expect this happens with the ones that have a Runnable from an extension as top level object in the config

Describe the solution you'd like
The error message should be clear on what's happening:

The provided configuration file contains duplicated keys 'xxxx'

[nmatthews-asapp]

@jeremyasapp we validated this is not the case with our current ruamel.yaml usage in v0.5, yes?

[jeremyasapp]

Provided we use version > 0.16.0