Print niv & flake "Importing" lines via logging and add error detail

This adds more details on import errors whenever they occur.
Previously we'd only print the first part of the error chain and thus
the user wouldn't get the error details that might be required for
fixing the error.

src/cli.rs

@@ -703,10 +703,13 @@ impl Opts {
             import(name, None, &mut pins, &niv).await?;
         } else {
             for (name, pin) in niv.iter() {
-                println!("Importing {}", name);
+                log::info!("Importing {}", name);
                 if let Err(err) = import(name, Some(pin), &mut pins, &niv).await {
                     log::error!("Failed to import pin '{}'", name);
                     log::error!("{}", err);
+                    err.chain()
+                        .skip(1)
+                        .for_each(|cause| log::error!("\t{}", cause));
                 }
             }
         }
@@ -798,10 +801,13 @@ impl Opts {
             .await?;
         } else {
             for (name, input_name) in inputs.iter() {
-                println!("Importing {}", name);
-                if let Err(err) = import(input_name, &mut pins, &nodes).await {
+                log::info!("Importing {}", name);
+                if let Err(err) = import(input_name, &mut pins, nodes).await {
                     log::error!("Failed to import pin '{}'", name);
                     log::error!("{}", err);
+                    err.chain()
+                        .skip(1)
+                        .for_each(|cause| log::error!("\t{}", cause));
                 }
             }
         }