Create friendlier errors for common problems

[KyleAMathews]

• EADDRINUSE should be caught and we explain another process is using it and how to find it
• If user runs gatsby develop in the wrong directory atm, Gatsby just throws an error about not being able to require app.js.
• fuzzy match unknown exports to APIs looking for typos and suggest the API they meant to use.
• If someone implements modifyWebpackConfig throw an error if the return isn't an object.

Suggest more in comments :)

[KyleAMathews]

Added invariant for return value of modifyWebpackConfig in #300

[stale]

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