providers/google: Don't fail deleting disks that don't exist.

[evandbrown]

Addresses #5942

[evandbrown]

Acceptance Test output:

$ make testacc TEST=./builtin/providers/google GOOGLE_PROJECT=evandbrown17 GOOGLE_REGION=us-central1 TESTARGS='-run=TestAccComputeInstance_disksWithA'                                                                               
==> Checking that code complies with gofmt requirements...
/home/evanbrown/dev/go-workspaces/terraform/bin/stringer
go generate $(go list ./... | grep -v /vendor/)
2016/05/16 11:52:50 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/google -v -run=TestAccComputeInstance_disksWithA -timeout 120m
=== RUN   TestAccComputeInstance_disksWithAutodelete
--- PASS: TestAccComputeInstance_disksWithAutodelete (90.59s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/google 90.695s

[jen20]

LGTM - thanks @evandbrown!

[ghost]

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