babelrc-related bugfixes

[mitranim]

• don't try to load missing babelrc (false check was too narrow)
• don't swallow non-FS errors when checking if file exists

[danez]

That completely changes the behaviour for when not setting babelrc option.

[danez]

Can you describe what problem you actually try to resolve?

[mitranim]

The problematic scenario is very basic, I'm surprised other people don't complain.

• webpack@3.3.0 + babel-loader@7.1.1
• .babelrc in project root
• rudimentary webpack config with use: {loader: 'babel-loader'} and no plugin options
• run webpack --watch, touch a file → webpack blows up and dies

Specifically: babel-loader attempts to check for existence of babelrc with the undefined name (not a string, just undefined), passing it to Webpack's virtual filesystem, which assumes that the file name is a string and produces a synchronous exception.

Checking for false is naive. By default, most users won't specify a babelrc location, meaning the option is undefined. The change accommodates the most common use case while keeping the same general logic.

[zythum]

because enhanced-resolve updated api. if exist
path is undefined will threw error webpack/enhanced-resolve#100

[danez]

Merged in #495