Add context to object storage error message

If object storage is not enabled for a certain type of object, project
exports may see "Object storage is not enabled". It's hard to know what
type of object is not enabled, so add the class name of the uploader to
see.

This came out of an attempt to run the project export in Kubernetes.

app/uploaders/object_storage.rb

@@ -394,7 +394,7 @@ module ObjectStorage
     def storage_for(store)
       case store
       when Store::REMOTE
-        raise 'Object Storage is not enabled' unless self.class.object_store_enabled?
+        raise "Object Storage is not enabled for #{self.class}" unless self.class.object_store_enabled?
 
         CarrierWave::Storage::Fog.new(self)
       when Store::LOCAL

spec/uploaders/object_storage_spec.rb

@@ -272,7 +272,7 @@ describe ObjectStorage do
             end
 
             it "to raise an error" do
-              expect { subject }.to raise_error(/Object Storage is not enabled/)
+              expect { subject }.to raise_error(/Object Storage is not enabled for JobArtifactUploader/)
             end
           end