Serializer: for Marshal, catch all .load errors

In order to avoid missing an error message to filter out, treat any Marshal.load error as a
failed serialization, and trust Ruby's e.cause system to provide a
lineage of the error's true beginning.

lib/dalli/protocol/value_serializer.rb

@@ -44,7 +44,20 @@ def store(value, req_options, bitflags)
 
       def retrieve(value, bitflags)
         serialized = (bitflags & FLAG_SERIALIZED) != 0
-        serialized ? serializer.load(value) : value
+        if serialized
+          if serializer.is_a?(Marshal)
+            begin
+              serializer.load(value)
+            rescue => e
+              raise UnmarshalError, "Unable to unmarshal value: #{e.message}"
+            end
+          else
+            # Use Dalli's existing exception filtering for deserialization when not using Marshal to serialize.
+            serializer.load(value)
+          end
+        else
+          value
+        end
       rescue TypeError => e
         filter_type_error(e)
       rescue ArgumentError => e

test/protocol/test_value_serializer.rb

@@ -158,6 +158,7 @@
 
     describe 'when the bitflags specify serialization' do
       it 'should deserialize the value' do
+        serializer.expect :is_a?, true, [Marshal]
         serializer.expect :load, deserialized_dummy, [raw_value]
         bitflags = rand(32)
         bitflags |= 0x1