Prevent `pijul apply` from creating empty channels

Dependencies

• [2] R3H7D42U Debugging `pijul git`: proper error reporting
• [3] SXEYMYF7 Fixing the bad changes in history (unfortunately, by rebooting).

Change contents

• replacement in pijul/src/commands/apply.rs at line 37
  [3.195267]→[2.723:778](∅→∅)
  − txn.open_or_create_channel(&channel_name)?
  [3.195267]

  [3.195402]
  + bail!("Channel {:?} not found", channel_name)