main: Improve error messages from Tera

The `to_string()` method of `tera::Error` structures only returns the
string value for itself; the messages from the underlying errors are
not included.  This effectively means any error from Tera just becomes
"failed to render template."  To provide a complete message, we have to
recursively call `source()` on the error chain and append its string
representation to the error message.

src/main.rs

@@ -132,7 +132,8 @@ fn process_instructions(
         let out = match tera.render(&i.template, &ctx) {
             Ok(o) => o,
             Err(e) => {
-                error!("Failed to render template {}: {}", i.template, e);
+                let msg = format_error(&e);
+                error!("Failed to render template {}: {}", i.template, msg);
                 continue;
             }
         };
@@ -229,3 +230,13 @@ fn chmod(
     info!("Set mode of {} to {:o}", path.as_ref().display(), newmode);
     Ok(())
 }
+
+fn format_error(e: &dyn std::error::Error) -> String {
+    // TODO replace this with std::error::Error::sources when it is stablized.
+    // https://github.com/rust-lang/rust/issues/58520
+    let mut msg = e.to_string();
+    if let Some(e) = e.source() {
+        msg.push_str(&format!(": {}", format_error(e)));
+    }
+    msg
+}