KnownTypePolymorhicMessagePackSerializer error message improvement

[thomas-joiner]

This applies to release 0.6.0-Beta4.

I ran into an issue when using KnownTypes. In the end, the issue ended up being that I was using [MessagePackKnownType] instead of [MessagePackKnownCollectionItemType]. The error message that I received was:

System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
   at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
   at MsgPack.Serialization.Polymorphic.KnownTypePolymorhicMessagePackSerializer`1.PackToCore(Packer packer, T objectTree)
[...]

It would have been nice to know what key it was looking up to make figuring out what the issue was easier.

Also, the typo in the class name should probably be fixed.

[yfakariya]

Fixed in 0.6.0-beta5.