fix: log to stderr, only color if no tty

Closes #87.

src/handler.rs

@@ -265,7 +265,7 @@ impl EventHandler for Handler {
             Ok(m) => m,
             Err(e) => {
                 // We can't return an error, but we also can't proceed, so log the error
-                // and bare-return to bail out.
+                // and early-return to bail out.
                 tracing::error!(%e, "failed to get message info for message");
                 return;
             }

src/tracing.rs

@@ -1,8 +1,12 @@
+use std::io::IsTerminal as _;
 use tracing_subscriber::{prelude::*, EnvFilter};
 
 /// Initializes a tracing subscriber.
 pub(crate) fn init_subscriber() -> anyhow::Result<()> {
-    let fmt_layer = tracing_subscriber::fmt::layer().with_target(true);
+    let fmt_layer = tracing_subscriber::fmt::layer()
+        .with_target(true)
+        .with_ansi(std::io::stderr().is_terminal())
+        .with_writer(std::io::stderr);
     let filter_layer = EnvFilter::try_from_default_env()
         .or_else(|_| EnvFilter::try_new("info"))?
         // Force disabling of r1cs log messages, otherwise the `ark-groth16` crate