Better error reporting (#95)

The main change is to print the whole error chain. We can combine this
with adding `.context` to distinct phases to be able to locate crashes
without having to use a debugger.

crates/puffin-cli/src/commands/sync.rs

@@ -1,7 +1,7 @@
 use std::fmt::Write;
 use std::path::Path;
 
-use anyhow::Result;
+use anyhow::{Context, Result};
 use bitflags::bitflags;
 use colored::Colorize;
 use itertools::{Either, Itertools};
@@ -206,7 +206,8 @@ pub(crate) async fn sync(
 
         let unzips = unzipper
             .download(downloads, cache.unwrap_or(staging.path()))
-            .await?;
+            .await
+            .context("Failed to download and unpack wheels")?;
 
         let s = if unzips.len() == 1 { "" } else { "s" };
         writeln!(

crates/puffin-cli/src/main.rs

@@ -159,7 +159,11 @@ async fn main() -> ExitCode {
         Err(err) => {
             #[allow(clippy::print_stderr)]
             {
-                eprintln!("{}: {}", "error".red().bold(), err);
+                let mut causes = err.chain();
+                eprintln!("{}: {}", "error".red().bold(), causes.next().unwrap());
+                for err in causes {
+                    eprintln!("  {}: {}", "Caused by".red().bold(), err);
+                }
             }
             ExitStatus::Error.into()
         }