Checking the HTTP Response isn't nil. Fixes #200

[tombuildsstuff]

In the Read function for most resources we check the http response code before raising an error (to check if it's a 404 not found) - however issue #200 has highlighted an issue where it's possible for the response to be nil if the connection doesn't return a response / is dropped - which currently crashes.

This PR does two things:

1. update the references to use a common validation method
2. checks the Responses Response object isn't nil before checking the status code is a 404

I'm running the entire test suite now - but the Virtual Network tests have run through successfully:

$ TF_ACC=1 envchain azurerm go test ./azurerm -v -timeout 120m -run TestAccAzureRMVirtualNetwork
=== RUN   TestAccAzureRMVirtualNetworkPeering_importBasic
--- PASS: TestAccAzureRMVirtualNetworkPeering_importBasic (140.31s)
=== RUN   TestAccAzureRMVirtualNetwork_importBasic
--- PASS: TestAccAzureRMVirtualNetwork_importBasic (89.01s)
=== RUN   TestAccAzureRMVirtualNetworkPeering_basic
--- PASS: TestAccAzureRMVirtualNetworkPeering_basic (140.09s)
=== RUN   TestAccAzureRMVirtualNetworkPeering_disappears
--- PASS: TestAccAzureRMVirtualNetworkPeering_disappears (123.54s)
=== RUN   TestAccAzureRMVirtualNetworkPeering_update
--- PASS: TestAccAzureRMVirtualNetworkPeering_update (167.10s)
=== RUN   TestAccAzureRMVirtualNetwork_basic
--- PASS: TestAccAzureRMVirtualNetwork_basic (76.22s)
=== RUN   TestAccAzureRMVirtualNetwork_disappears
--- PASS: TestAccAzureRMVirtualNetwork_disappears (89.03s)
=== RUN   TestAccAzureRMVirtualNetwork_withTags
--- PASS: TestAccAzureRMVirtualNetwork_withTags (105.97s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm	931.292s

Fixes #200

[mbfrahry]

LGTM!

[ghost]

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