mutable/retrieve.py: clean up control flow to avoid dropping errors

mutable/retrieve.py: clean up control flow to avoid dropping errors

* replace DeferredList with gatherResults, simplify result handling
* use BadShareError to signal recoverable problems in either fetch or
  validate, catch after _validate_block
* _validate_block is thus not responsible for noticing fetch problems
* rename _validation_or_decoding_failed() to _handle_bad_share()
* _get_needed_hashes() returns two Deferreds, instead of a hard-to-unpack
  DeferredList