refactor error handling to use thiserror

[ahdinosaur]

did as part of ssb-archive

• changed error enums from using failure to using thiserror.
  • this changes what the functions return, previously failure::Error, now they return an error enum, e.g. FlumeOffsetLogError.
• added error variants for source errors that were previously handled automatically
  • i decided to make specific variants for each situation, rather than for example just having an Io(#[from] std::io::Error) variant.
• changed the FlumeLog trait to take an associated type Error.

open to feedback on the best way to do this.