[31m[1mERROR[0m[39m: 
  [31m×[0m Error observers must have no return type.
  [31m│[0m `app::error_observer` returns `alloc::string::String`.
  [31m│[0m
  [31m│[0m     ╭─[[36;1;4msrc/lib.rs[0m:9:1]
  [31m│[0m  [2m 9[0m │     let mut bp = Blueprint::new();
  [31m│[0m  [2m10[0m │     bp.error_observer(f!(crate::error_observer));
  [31m│[0m     · [35;1m                      ────────────┬────────────[0m
  [31m│[0m     ·                   [35;1mThe error observer was registered here[0m
  [31m│[0m  [2m11[0m │     bp
  [31m│[0m     ╰────