Throw msg exception before build nonpublic formatter.

[neuecc]

when serializing non-public type, throws an unfriendly exception.

System.TypeLoadException: Type 'MessagePack.Formatters.Sandbox_ClassWithMissingKeyPositionsFormatter1' from assembly 'MessagePack.Resolvers.DynamicObjectResolver, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' is attempting to implement an inaccessible interface.
   at System.Reflection.Emit.TypeBuilder.TermCreateClass(RuntimeModule module, Int32 tk, ObjectHandleOnStack type)
   at System.Reflection.Emit.TypeBuilder.CreateTypeNoLock()
   at System.Reflection.Emit.TypeBuilder.CreateTypeInfo()

so check the access modifier before build formatter and throws friendly msg.

MessagePack.MessagePackSerializationException: Building dynamic formatter only allows public type. Type: Sandbox.ClassWithMissingKeyPositions

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This PR includes modifications to Directory.Build.targets that have nothing to do with this title.
when edit code for Unity, I'm removing Directory.Build.props to prevent error msgs. related to #556
Then, bin and obj are generated under each project, and it is read, so it causes compile error.
Unnecessary .meta is also loaded into SolutionExplorer.
This modifies of Directory.Build.Targets is to remove the there unnecessary file.

[neuecc]

@AArnott
#660 is very good but still requires an accessibility check also after implementing it if standard resolver does not allow private.
I pushed the code that I corrected, so I requested for a review again.