provider/aws: Fix DB Subnet refresh issue

[catsby]

Fixes #496 by updating the read function to mark the resource as deleted if not found.

[mitchellh]

LGTM.

[dharmapunk82]

I'm hitting this on latest 0.7.1 running on ubuntu 14.04, for the eip_association resource. Lost network during a destroy then was not able to plan, apply, or taint the 2 resources in question.

I manually deleted the resources from tfstate and was then able to destroy the rest of the infrastructure, but wanted to report the bug... Plan and apply both failed with errors below prior to manually deleting them from tfstate:

Error refreshing state: 2 error(s) occurred:

• aws_eip_association.ngomez_eip_assoc.1: Unable to find EIP Association: eipassoc-56135930
• aws_eip_association.ngomez_eip_assoc.0: Unable to find EIP Association: eipassoc-fd13599b

Attempting to taint:
terraform taint eipassoc-56135930
Failed to parse resource name: Malformed resource state key: eipassoc-56135930

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

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