Add warnings and improve error formatting

[ShaddyDC]

Current output is workable, but it's not great:

› echo "   \n\n\n\n\n" | cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s
     Running `target/debug/committable`
Error:   × 2 error(s)

Error:   × NonEmptyHeader
   ╭─[1:1]
 1 │    
   · ─┬─
   ·  ╰── header is empty
 2 │ 
   ╰────
Error:   × NonEmptyBody
   ╭─[2:1]
 1 │        
 2 │ ╭─▶ 
 3 │ │   
 4 │ │   
 5 │ ├─▶ 
   · ╰──── body is empty
   ╰────

Some points on better use of warnings and groups are here and here.
I'll have to think about how I want to go about this.