Catch KeyError when filtering errors from parse-conda-requirements.py
This PR improves the error handling in the parse-conda-requirements.py
script. When an UnsatisfiableError is raised by conda install
there won't be a 'packages'
key in the JSON output, so we need to catch KeyError
then run it again to expose the underlying problem. See build errors in #1366 for an example of why this is needed.