pijul channel delete: error if channel doesn't exist

Dependencies

• [2] FMKKWCFV Better clap attributes
• [3] SXEYMYF7 Fixing the bad changes in history (unfortunately, by rebooting).
• [4] VQPAUKBQ channel switch as an alias to reset
• [*] I52XSRUH Massive cleanup, and simplification

Change contents

• edit in pijul/src/commands/channel.rs at line 5
  [6.6264]

  [2.147]
  + use anyhow::anyhow;
• replacement in pijul/src/commands/channel.rs at line 58
  [3.188305]→[3.188305:188348](∅→∅)
  − txn.drop_channel(delete)?;
  [3.188305]

  [3.188348]
  + if !txn.drop_channel(delete)? {

  + return Err(anyhow!("Channel {} not found", delete));

  + }