pull: include the patch output when a .diff.gz fails to apply

patch reports rejected hunks on stdout, not stderr: the error used
to carry only the diff path and an empty stderr, naming neither the
failing file nor the reason.

src/pull.rs

@@ -779,10 +779,16 @@ async fn fetch_archive_sources(
                 .output()?;
 
             if !output.status.success() {
+                // patch reports rejected hunks on stdout, not stderr
+                let stdout = String::from_utf8_lossy(&output.stdout);
                 let stderr = String::from_utf8_lossy(&output.stderr);
-                return Err(
-                    format!("Failed to apply patch: {}\n{}", diff_path.display(), stderr).into(),
-                );
+                return Err(format!(
+                    "Failed to apply patch: {}\n{}{}",
+                    diff_path.display(),
+                    stdout,
+                    stderr
+                )
+                .into());
             }
 
             debug!("Successfully applied patch: {}", diff_path.display());