provider/azure: Provide a simpler error when using a Platform Image without a Storage Service

[catsby]

Currently, when attempting to use a Platform Image without a Storage Service, you get a cryptic and misleading error:

Error applying plan:

1 error(s) occurred:

* azure_instance.web: Could not find image with label 'OpenLogic 6.5'. Available images are: CDH 5.1 Evaluation, Microsoft Azure Site Recovery Configuration Server, Microsoft Azure Site Recovery Configuration Server, Microsoft Azure Site Recovery Master Target Server, Microsoft Azure Site Recovery Master Target Server, Microsoft Azure Site Recovery Process Server, Microsoft Azure Site Recovery Process Server, HDP 2.1.5 with 16 data disks, HDP 2.1.5 with 8 data disks, Hortonworks Sandbox with HDP 2.2.4-2, Hortonworks Sandbox with HDP 2.2, Hortonworks Sandbox with HDP 2.2.4-2v2, Hortonworks Sandbox with HDP 2.2.4-2v3, Hortonworks Sandbox with HDP 2.2.4-2v4, Hortonworks Sandbox with HDP 2.2.4-2v5, ...... 

The error is originating here:

• https://github.com/hashicorp/terraform/blob/master/builtin/providers/azure/resource_azure_instance.go#L642-668

But we're swallowing that specific error and returning a more generic one here:

• https://github.com/hashicorp/terraform/blob/master/builtin/providers/azure/resource_azure_instance.go#L589-L595

This obfuscates what the underlying problem is and the user can't self-diagnose and correct.

My patch does a few things, and we can pick what we like:

• Adds a note to the documentation that Platform Images require a Storage Service
• Introduces a generic errors file to consolidate some common errors, but only has a PlatformStorageError for now
• Checks for PlatformStorageError after retrieveOSImageDetails, and returns that error afterwards

New output:

Error applying plan:

1 error(s) occurred:

* azure_instance.web: When using a platform image, the 'storage' parameter is required

I've introduced the specific errors here to help trap any errors thrown but also maintain the current fall-through flow in retrieveImageDetails (if retrieveVMImageDetails errors, then still try retrieveOSImageDetails, and error at the end).

[catsby]

cc @svanharmelen and @aznashwan for some Azure provider input, I'm new to it 😄

[svanharmelen]

LGTM!

PR #2607 is likely going to change this part even more to make it even more user friendly (through it's a bit silent there lately), but until that PR is done and merged, this is a good intermediate step I think.

[svanharmelen]

Extremely small nit, but since the line is now way shorter there's not much reason to place PlatformStorageError on it's own line... Through it doesn't hurt either of course! 😉

[catsby]

For sure, it's better on one line 😄 , done in 4885874

[ghost]

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