Raise an error when a malformed package.json file is found.

Raise an error when a malformed package.json file is found.

The current behaviour will silently ignore any parsing errors
that may occur when loading a package.json file. This makes
debugging errors in the package.json file very difficult.

This changes the behaviour that that errors opening and reading
the file package.json file continue to be ignored, but errors
in parsing will throw an exception.