Fix subtle py3 bug in t.m.pop3 detected by higher coverage.

Before this, protocol error messages would send literal "b'error message'" to
the clients.

src/twisted/mail/pop3.py

@@ -575,8 +575,9 @@ class POP3(basic.LineOnlyReceiver, policies.TimeoutMixin):
             return self.processCommand(*line.split(b' '))
         except (ValueError, AttributeError, POP3Error, TypeError) as e:
             log.err()
-            self.failResponse('bad protocol or server: {}: {}'.format(
-                e.__class__.__name__, e))
+            self.failResponse(b': '.join([b'bad protocol or server',
+                                          e.__class__.__name__.encode('utf-8'),
+                                          b''.join(e.args)]))
 
 
     def processCommand(self, command, *args):

src/twisted/mail/test/test_pop3.py

@@ -599,9 +599,7 @@ class AnotherPOP3Tests(unittest.TestCase):
         Sending a command with invalid UTF-8 characters
         will raise a L{pop3.POP3Error}.
         """
-        error = str(b'not authenticated yet: cannot do \x81PASS')
-        if not isinstance(error, bytes):
-            error = error.encode("utf-8")
+        error = b'not authenticated yet: cannot do \x81PASS'
         d = self.runTest(
             [b'\x81PASS',
              b'QUIT'],