Improve error logging in the visualisation code

Change-Id: I607e732121cfa357cef3dea167e0adae7d6ac61c
diff --git a/visualisation/visualisation.py b/visualisation/visualisation.py
index 3f0c75c..5df9163 100644
--- a/visualisation/visualisation.py
+++ b/visualisation/visualisation.py
@@ -244,9 +244,10 @@
         # We've been asked for a VLAN that doesn't exist
         if vlan is None:
             self.send_response(404)
-            self.wfile.write('Content-type: text/html\r\n')
+            self.wfile.write('Content-type: text/plain\r\n')
             self.end_headers()
-            self.wfile.write('404 Not Found')
+            self.wfile.write('404 Not Found\r\n')
+            logging.error('VLAN graphic not found - asked for %s', self.parsed_path.path)
             return
 
         gim = Graphics()
@@ -259,9 +260,11 @@
         # If we can't get the font we need, fail
         except NameError:
             self.send_response(500)
-            self.wfile.write('Content-type: text/html\r\n')
+            self.wfile.write('Content-type: text/plain\r\n')
             self.end_headers()
-            self.wfile.write('500 Internal Server Error')
+            self.wfile.write('500 Internal Server Error\r\n')
+            logging.error('Unable to generate graphic, no fonts found - asked for %s',
+                          self.parsed_path.path)
             return
 
         switch = {}
@@ -351,9 +354,10 @@
 
         # Fall-through for any files we don't recognise
         self.send_response(404)
-        self.wfile.write('Content-type: text/html\r\n')
+        self.wfile.write('Content-type: text/plain\r\n')
         self.end_headers()
         self.wfile.write('404 Not Found')
+        logging.error('File not supported - asked for %s', self.parsed_path.path)
         return
 
     # Override the BaseHTTPRequestHandler log_message() method so we