Fix error message with .eslint syntax error

[ColCh]

This error is unhelpful

> plato -d plato -r --eslint .eslintrc src

undefined:15
  },
  ^

SyntaxError: Unexpected token }
    at Object.parse (native)
    at Object.exports.exec (/my-project/node_modules/plato/lib/cli.js:49:21)
    at Object.<anonymous> (/my-project/node_modules/plato/bin/plato:19:5)
    at Module._compile (module.js:413:34)
    at Object.Module._extensions..js (module.js:422:10)
    at Module.load (module.js:357:32)
    at Function.Module._load (module.js:314:12)
    at Function.Module.runMain (module.js:447:10)
    at startup (node.js:140:18)
    at node.js:1001:3