Fix a boost format string abort

Dependencies

• [2] RX6UB7YW Better AWS error messages
• [3] GTUZLZRH Add an S3-backed binary cache store
• [4] SOB276BA Keep some statistics for the binary cache stores

Change contents

• replacement in src/hydra-queue-runner/s3-binary-cache-store.cc at line 111
  [3.3917]→[2.702:790](∅→∅)
  − throw Error(format("AWS error fetching ‘%s’") % path % error.GetMessage());
  [3.3917]

  [3.3985]
  + throw Error(format("AWS error fetching ‘%s’: %s") % path % error.GetMessage());