Report failure to read a json file

With an invalid package.json, such as an empty one, `slc strongops
--package` was reporting errors such as:

   Error writing to ./package.json: {}

After this change, the error is:

  Error writing to './package.json': read failed (Unexpected end of input)
Merge pull request #103 from strongloop/feature/fix-sln-721

Report failure to read a json file