[gatsby-source-filesystem] Validate the existence of path in options property

[shri3k]

While specifying path in gatsby-config.js validate the path and let the user know if it doesn't exist instead of failing quietly

e.g.

module.exports = {
  siteMetadata: {
    title: `Pandas Eating Lots`,
  },
  plugins: [
    {
      resolve: `gatsby-source-filesystem`,
      options: {
        name: `src`,
        path: `${__dirname}/src/some_path`, // some_path doesn't exist
      },
    },
  ],
}

> gatsby develop

Everything feels like it's working so far but in GraphiQL we find that allFile query will fail indicating something's not right.

I think it'd be helpful to let the user know that the path is invalid while (re)building but will defer to the team on what the right solution should be.