More visible telnet conch message

Capture traceback when trying to import required twisted modules,
print it in case telnet is enabled, and mention settings variable
that can be used to supress the message.
Thanks @kmike!

scrapy/extensions/telnet.py

@@ -6,6 +6,7 @@ See documentation in docs/topics/telnetconsole.rst
 
 import pprint
 import logging
+import traceback
 
 from twisted.internet import protocol
 try:
@@ -13,6 +14,7 @@ try:
     from twisted.conch.insults import insults
     TWISTED_CONCH_AVAILABLE = True
 except (ImportError, SyntaxError):
+    _TWISTED_CONCH_TRACEBACK = traceback.format_exc()
     TWISTED_CONCH_AVAILABLE = False
 
 from scrapy.exceptions import NotConfigured
@@ -40,8 +42,9 @@ class TelnetConsole(protocol.ServerFactory):
         if not crawler.settings.getbool('TELNETCONSOLE_ENABLED'):
             raise NotConfigured
         if not TWISTED_CONCH_AVAILABLE:
-            raise NotConfigured('TelnetConsole not enabled: failed to import '
-                                'required twisted modules.')
+            raise NotConfigured(
+                'TELNETCONSOLE_ENABLED setting is True but required twisted '
+                'modules failed to import:\n' + _TWISTED_CONCH_TRACEBACK)
         self.crawler = crawler
         self.noisy = False
         self.portrange = [int(x) for x in crawler.settings.getlist('TELNETCONSOLE_PORT')]