Improved JSON parsing error messaging

[dougbtv]

Feng mentioned during a recent demo that one thing that might not be intuitive to users is how to fix an error that's caused by a JSON parsing error, with an error message such as:

error parsing configuration: invalid character ':' after top-level value

Two thoughts regarding this:

1. We could potentially wrap / trap this error and give a more general message, maybe something along the lines of "JSON parsing error in config: field, please check that there's valid JSON within"
2. We could go upstream and update the CNI libraries to provide some better messaging.

[dcbw]

@dougbtv we should certainly do #2 regardless