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

test: Error w/ cause case

fix: recursively find Error.cause stacks

Remove the error.stack logic as it is redundant

test: check for both 1 level & 2 level Error.cause

refactor: use native util.format() API for Error printing

fix: put back original lines of code

test: update tests to be less brittle

index.js

@@ -11,6 +11,7 @@
  * @private
  */
 
+var util = require('util')
 var debug = require('debug')('finalhandler')
 var encodeUrl = require('encodeurl')
 var escapeHtml = require('escape-html')
@@ -172,8 +173,8 @@ function getErrorMessage (err, status, env) {
   var msg
 
   if (env !== 'production') {
-    // use err.stack, which typically includes err.message
-    msg = err.stack
+    // use util.format to get native Error print
+    msg = util.format(err)
 
     // fallback to err.toString() when possible
     if (!msg && typeof err.toString === 'function') {

test/test.js

@@ -650,6 +650,39 @@ var topDescribe = function (type, createServer) {
         })
     })
   })
+  describe('Errors with cause', function () {
+    it('should return Error message with 1 level cause trace', function (done) {
+      var err = new Error('foo', { cause: new Error('bar') })
+      var expectedRegex = [/Error: foo/, /\[cause\]: Error: bar/];
+
+      request(createServer(err))
+        .get('/')
+        .then((response) => {
+          for (var i = 0; i < expectedRegex.length; ++i) {
+            var regex = expectedRegex[i];
+            assert.match(response.text, regex);
+          }
+          done()
+        })
+        .catch(done)
+    })
+
+    it('should return Error message with 2 level cause trace', function (done) {
+      var err = new Error('foo', { cause: new Error('bar', { cause: new Error('baz') }) })
+      var expectedRegex = [/Error: foo/, /\[cause\]: Error: bar/, /\[cause\]: Error: baz/];
+
+      request(createServer(err))
+        .get('/')
+        .then((response) => {
+          for (var i = 0; i < expectedRegex.length; ++i) {
+            var regex = expectedRegex[i];
+            assert.match(response.text, regex);
+          }
+          done()
+        })
+        .catch(done)
+    })
+  })
 }
 
 var servers = [