better handling for error cases in FileMetadata.find

the base case originally tested relied on a bug in
Valkyrie (samvera/valkyrie#905).

this reimplements with a more serious error handling approach.

app/models/hyrax/file_metadata.rb

@@ -155,15 +155,17 @@ def content
       ''
     end
 
+    ##
+    # @return [Valkyrie::StorageAdapter::File]
+    #
+    # @raise [Valkyrie::StorageAdapter::AdapterNotFoundError] if no adapter
+    #   could be found matching the file_identifier's scheme
+    # @raise [Valkyrie::StorageAdapter::FileNotFound] when the file can't
+    #   be found in the registered adapter
     def file
-      adapter =
-        begin
-          Valkyrie::StorageAdapter.adapter_for(id: file_identifier)
-        rescue Valkyrie::StorageAdapter::AdapterNotFoundError => _err
-          Hyrax.storage_adapter
-        end
-
-      adapter.find_by(id: file_identifier)
+      Valkyrie::StorageAdapter
+        .adapter_for(id: file_identifier)
+        .find_by(id: file_identifier)
     end
   end
 end

spec/models/hyrax/file_metadata_spec.rb

@@ -117,7 +117,39 @@
 
   describe '#file' do
     it 'returns file from storage adapter' do
-      expect(subject.file).to be_a Valkyrie::StorageAdapter::StreamFile
+      expect(file_metadata.file).to be_a Valkyrie::StorageAdapter::StreamFile
+    end
+
+    context 'when file_identifier is saved with storage_adapter' do
+      let(:file_metadata) { described_class.new(file_identifier: stored.id) }
+      let(:file_set) { FactoryBot.valkyrie_create(:hyrax_file_set) }
+
+      let(:stored) do
+        Hyrax.storage_adapter.upload(resource: file_set,
+                                     file: file,
+                                     original_filename: file.original_filename)
+      end
+
+      it 'gives the exact file' do
+        expect(file_metadata.file.read).to eq stored.read
+      end
+    end
+
+    context 'when file_identifier is saved with an arbitrary adapter' do
+      let(:file_metadata) { described_class.new(file_identifier: stored.id) }
+      let(:file_set) { FactoryBot.valkyrie_create(:hyrax_file_set) }
+
+      let(:stored) do
+        Valkyrie::StorageAdapter
+          .find(:test_disk)
+          .upload(resource: file_set,
+                  file: file,
+                  original_filename: file.original_filename)
+      end
+
+      it 'gives the exact file' do
+        expect(file_metadata.file.read).to eq stored.read
+      end
     end
   end