use absolute path in error

[chriselion]

I think conda might do something weird with the working directory; there have been a few issues with users passing bad paths to mlagents-learn. This replaces the relative path in the exception with the absolute path, so hopefully it's easier to see where it's trying (and failing) to load from.

before:

mlagents.trainers.exception.TrainerConfigError: Config file could not be found at bad.yaml.

after:

mlagents.trainers.exception.TrainerConfigError: Config file could not be found at /Users/chris.elion/code/ml-agents/bad.yaml.

[harperj]

👍