Exception message could be improved

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Open the 
http://code.google.com/p/protobuf-net/source/browse/branches/v1/protobuf-net/Pro
perty/PropertyEnum.cs
2. Look at the code at Serialize()
3. Note that exception message includes DefaultValue (which is wrong!) - it 
looks like copy&paste from OnAfterInit (where it is correct)

What is the expected output? What do you see instead?
1. It is much better to include in exception message real enum value - 
GetValue(source)

What version of the product are you using? On what operating system?
1. v1 branch.

Please provide any additional information below.
1. Nothing.

Original issue reported on code.google.com by vadim.sk...@gmail.com on 15 Jul 2011 at 3:00