Expose Ack Timeout length for unit test writing
Added CRC checking
Added the ability to simulate server errors

Also changed the downloaded file contents to not include the file
length in the first byte. This makes the code a bit less confusing.
Test FileManager against simulated server errors

Specifically much better testing for the handling of:
- Ack responses not being sent by server
- Getting Nak’s back from commands
- CRC failures

Also changed file validation to take into account Mock Server no longer
sending file length in first byte of file.
Merge pull request #790 from DonLakeFlyer/BetterFileMgrUt

More comprehensive testing of FileManager error/edge cases