core: raise better errors on common logging block syntax mistakes

These are commonly reported and include:
- Commenting out the contents of logging:channels without commenting out the "channels:" heading, causing that block to become None.
- Commenting out headers like "filerotation:", causing its body to become pairs in logging:files or something similar.
- Leaving logging:channels:<netname> empty: this causes it to become None, so using get() on it fails.

classes.py

@@ -90,20 +90,27 @@ def logSetup(self):
         """
         try:
             channels = conf.conf['logging']['channels'][self.name]
-        except KeyError:  # Not set up; just ignore.
+        except (KeyError, TypeError):  # Not set up; just ignore.
             return
 
         log.debug('(%s) Setting up channel logging to channels %r', self.name,
                   channels)
 
+        # Only create handlers if they haven't already been set up.
         if not self.loghandlers:
-            # Only create handlers if they haven't already been set up.
+            if not isinstance(channels, dict):
+                log.warning('(%s) Got invalid channel logging configuration %r; are your indentation '
+                            'and block commenting consistent?', self.name, channels)
+                return
 
             for channel, chandata in channels.items():
                 # Fetch the log level for this channel block.
                 level = None
-                if chandata is not None:
+                if isinstance(chandata, dict):
                     level = chandata.get('loglevel')
+                else:
+                    log.warning('(%s) Got invalid channel logging pair %r: %r; are your indentation '
+                                'and block commenting consistent?', self.name, filename, config)
 
                 handler = PyLinkChannelLogger(self, channel, level=level)
                 self.loghandlers.append(handler)

log.py

@@ -87,7 +87,11 @@ def stopFileLoggers():
 files = conf.conf['logging'].get('files')
 if files:
     for filename, config in files.items():
-        makeFileLogger(filename, config.get('loglevel'))
+        if isinstance(config, dict):
+            makeFileLogger(filename, config.get('loglevel'))
+        else:
+            log.warning('Got invalid file logging pair %r: %r; are your indentation and block '
+                        'commenting consistent?', filename, config)
 
 log.debug("log: Emptying log_queue")
 # Process and empty the log queue