Changing trait instance label names on resource definitions should be a breaking change

[mrjerryjohns]

Currently, altering the trait instance label name on a resource definition is flagged as a warning by the compiler:

E.g Changing the LabelSettingsTrait instance 'device_label' to 'device_label_moved':

WARNING:root:RemovedObjectValidator has validation warnings:
  Previous schema item weave.resource.service.DescriptionServiceResource.device_label is missing from current schema. Schema items cannot be removed without breaking backward compatibility.

On the service layer, this change is actually deemed a breaking change since the labels are used to index into their data storage layer.

Consequently, the WDL compiler should be modified to flag this appropriately.

[gerickson]

Before we move forward with this, was that indexing choice an ecosystem-specific implementation choice and, as such, should this be a -Werror=device-label-moved option that ecosystems can specific to the extent they made such a choice?