assert: show thrown message in doesNotThrow()

assert.doesNotThrow() should show actual error message instead
of "Got unwanted exception" which is not really helpful.

PR-URL: nodejs#12167
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>

lib/assert.js

@@ -546,7 +546,10 @@ function innerThrows(shouldThrow, block, expected, message) {
   } else if (actual !== undefined) {
     if (!expected || expectedException(actual, expected)) {
       details = message ? `: ${message}` : '.';
-      fail(actual, expected, `Got unwanted exception${details}`, fail);
+      fail(actual,
+           expected,
+           `Got unwanted exception${details}\n${actual.message}`,
+           fail);
     }
     throw actual;
   }

test/parallel/test-assert.js

@@ -468,6 +468,34 @@ assert.throws(() => {
 }, /Got unwanted exception: user message/,
               'a.doesNotThrow ignores user message');
 
+{
+  let threw = false;
+  try {
+    assert.doesNotThrow(makeBlock(thrower, Error), 'user message');
+  } catch (e) {
+    threw = true;
+    common.expectsError({
+      code: 'ERR_ASSERTION',
+      message: /Got unwanted exception: user message\n\[object Object\]/
+    })(e);
+  }
+  assert.ok(threw);
+}
+
+{
+  let threw = false;
+  try {
+    assert.doesNotThrow(makeBlock(thrower, Error));
+  } catch (e) {
+    threw = true;
+    common.expectsError({
+      code: 'ERR_ASSERTION',
+      message: /Got unwanted exception\.\n\[object Object\]/
+    })(e);
+  }
+  assert.ok(threw);
+}
+
 // make sure that validating using constructor really works
 {
   let threw = false;