Add GrainException to support better error handling from generated grains

[benbenwilde]

Add GrainException which enables throwing a GrainException back on the grain client with a specific message and code from inside a grain message handler. This enables application code to easily provide different error handling behavior based on the code.

For example, if i want to throw an error inside of a grain handler due to bad user input. Right now it just throws a plain exception on the client. With GrainException and especially the new Code property, we can pass something to indicate that the error was due to user input, and not some other unknown error in the code.

This change is fully backwards compatible with existing generated grains and usage.

Description

Purpose

This pull request is a:

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)