developer server crash when deleting markdown directories

[goofytroopy]

Deleting directory make gatsby crash. When working with markdownfiles and deleting a directory the developer server will crash. The same check that is for files should also be on directories, otherwise it might try to delete a directory that does not exist.

[pieh]

That's good catch! Thanks @goofytroopy!

[gatsbot]

Holy buckets, @goofytroopy — we just merged your PR to Gatsby! 💪💜

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