feat: handle err.cause & err.stack

This is a replacement for pillarjs#49.
I just pulled out the specific change related to this issue and added it here.

Closes: expressjs/express#5630

index.js

@@ -175,8 +175,12 @@ function getErrorMessage (err, status, env) {
     // use err.stack, which typically includes err.message
     msg = err.stack
 
-    // fallback to err.toString() when possible
-    if (!msg && typeof err.toString === 'function') {
+    if (err && err.cause && err.cause.stack) {
+      msg += '\n[cause]: ' + err.cause.stack
+    } else if (err && err.stack) {
+      msg += '\n[stack]: ' + err.stack
+    } else if (!msg && typeof err.toString === 'function') {
+      // fallback to err.toString() when possible
       msg = err.toString()
     }
   }