provider/aws: Handle the case where Route Table is already gone.

[kwilczynski]

This commit changes the behaviour of the ExistsFunc, where by default
lack of a route table (e.g. already removed, etc.) would cause an error
to be thrown. This makes is hard to carry out any action e.g. plan,
refresh, or destroy, that rely on the route table existance check.

Also, make error messages a little better in terms of wording, etc.

Signed-off-by: Krzysztof Wilczynski krzysztof.wilczynski@linux.com

[kwilczynski]

Resolves #9421.

[kwilczynski]

The acceptance test is passing:

$ make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSRoute_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2016/10/18 11:43:51 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSRoute_ -timeout 120m
=== RUN   TestAccAWSRoute_basic
--- PASS: TestAccAWSRoute_basic (72.78s)
=== RUN   TestAccAWSRoute_changeCidr
--- PASS: TestAccAWSRoute_changeCidr (112.46s)
=== RUN   TestAccAWSRoute_noopdiff
--- PASS: TestAccAWSRoute_noopdiff (191.93s)
=== RUN   TestAccAWSRoute_doesNotCrashWithVPCEndpoint
--- PASS: TestAccAWSRoute_doesNotCrashWithVPCEndpoint (84.61s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    461.799s

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