EncodingAction Error Messages

Added proper error messages to the exceptions thrown by the EncodingAction class.
ASCII Unit Test Clean Up

Cleaned up some style/readability issues in the ASCII unit tests.
UTF8 Unit Tests

Added more UTF8 unit tests:

- Verify size of UTF-8 code unit
- Verify UTF-8 BOM
- Verify UTF8::CanRepresent and UTF8::Count
- Verify UTF-8 encoding
UTF8 Unit Test Fix

Fixed a typo in a UTF8 unit test.