Avoid logging an object on compilation errors
Avoid logging an object on compilation errors

In certain configurations Storybook might log the whole Webpack
build object when warnings or errors are thrown. This was caused
by a `logger.error` call inside the `index.js` catch. This fixes by logging
the error only when it's an instance of an error.
storybooks/react-storybook,storybooks/storybook,rhalff/storybook,storybooks/react-storybook,kadirahq/react-storybook,kadirahq/react-storybook,rhalff/storybook,storybooks/react-storybook,storybooks/storybook,rhalff/storybook,storybooks/storybook,rhalff/storybook,storybooks/storybook,rhalff/storybook,storybooks/react-storybook,storybooks/storybook,rhalff/storybook,storybooks/storybook,storybooks/storybook