Fix Eltro handling of error messages so they always become normalized as Error objects

lib/eltro.mjs

@@ -189,7 +189,18 @@ Eltro.prototype.__runTest = async function(stats, test) {
     .then(function() {
         stats.passed++
       }, function(err) {
-        test.error = err
+        let saveError = err
+        if (!saveError) {
+          saveError = new Error('Test promise rejected with empty message')
+        } else if (typeof(saveError) !== 'object' || saveError.message == null || saveError.stack == null) {
+          try {
+            saveError = new Error('Test promise rejected with ' + JSON.stringify(saveError))
+          } catch (parseError) {
+            saveError = new Error('Test promise rejected with ' + saveError + ' (Error stringifying: ' + parseError.message + ')')
+          }
+          saveError.originalError = err
+        }
+        test.error = saveError
         stats.failed++
       }
     )

test/failure.test.mjs

@@ -0,0 +1,28 @@
+import e from '../lib/eltro.mjs'
+import assert from '../lib/assert.mjs'
+
+function CreateT() {
+  const t = new e.Eltro()
+  t.reporter = ''
+  return t
+}
+
+e.test('Eltro should support any value in promise fail', async function() {
+  const t = CreateT()
+  t.begin()
+  t.describe('', function() {
+    t.test('a', function() { return new Promise(function(res, rej) { rej() }) })
+    t.test('b', function() { return new Promise(function(res, rej) { rej(null) }) })
+    t.test('c', function() { return new Promise(function(res, rej) { rej(undefined) }) })
+    t.test('d', function() { return new Promise(function(res, rej) { rej(124523) }) })
+    t.test('e', function() { return new Promise(function(res, rej) { rej('testety') }) })
+  })
+  await t.run()
+  assert.strictEqual(t.failedTests.length, 5)
+
+  for (let x = 0; x < t.failedTests.length; x++) {
+    assert.strictEqual(typeof(t.failedTests[x].error), 'object')
+    assert.ok(t.failedTests[x].error.message)
+    assert.ok(t.failedTests[x].error.stack)
+  }
+})