Fixed Webpack stats parsing errors

[viktor-podzigun]

Notes:

• fixes webpack json stats parsing errors on missing optional fields in errors/warnings objects:

  
  [error] Error parsing webpack stats output
  [error] /warnings(0)/moduleName: JsonValidationError(List(error.path.missing),WrappedArray())
  [error] /warnings(1)/moduleName: JsonValidationError(List(error.path.missing),WrappedArray())

[error] Error parsing webpack stats output [error] /errors(0)/loc: JsonValidationError(List(error.path.missing),WrappedArray()) [error] /errors(1)/loc: JsonValidationError(List(error.path.missing),WrappedArray())

- in case other parsing errors occur - prints `webpack` command to run manually that will help fixing original issues
- related to #350

[cornerman]

Can we get this merged? This would definitely help when encountering warning/hints/errors. Current workaround is to ignore/silence these warnings in your webpack config.