Cleanup ExceptionConversionHandlers

[olegsych]

To improve readability of the code, make type names more descriptive:

• Rename the internal V2.Runtime.ExceptionConversionHandler to ExceptionSerializer
• Rename the internal V2.Client.ExceptionConversionHandler to ExceptionDeserializer

In types that were already affected by this change, perform additional opportunistic cleanup using primarily Visual Studio tooling and without changing the structure of the code.

To simplify code analysis, lock down internal types:

• Make internal types sealed, where possible
• Make unused members of internal types private
• Make fields readonly where possible
• Use IEnumerable<> instead of List<> to reduce the risk of unintended modification

To improve readability

• Use explicit type where variable type doesn't appear explicitly in the initialization statement
• Move using directives above namespace, consistently with the .NET coding style instead of inside the namespace style used with C++
• Remove generated comments that duplicate information already available in the code