provider/digitalocean: check if the droplet no longer exists.

[rgl]

if you manually delete a droplet from the Digital Ocean control panel, you no longer can use terraform without fiddling with the tfstate file due to:

terraform plan -no-color
Refreshing Terraform state prior to plan...

digitalocean_droplet.web: Refreshing state... (ID: 5874338)
Error refreshing state: 1 error(s) occurred:

* 1 error(s) occurred:

* Error retrieving droplet: Error retrieving droplet: API Error: 404 Not Found

The resource reader was missing a check for the droplet existence. this PR fixes that.

[phinze]

Thanks for this!

Sad that we have to match on the exact error, but I took a look at upstream and it's the best we can do for now.

[ghost]

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