[#24602] Netrender master node IP information is reset to [Default] no matter what

[#24601] Net rendering master node fails to send/retrieve files to/from slaves

Both Reported by Dan McGrath

The second might not actually be fixed with this, but now there is better error checking and reporting.

Misc: add poll method in Panel base class, to better support mixins with different inheritance order.

release/scripts/io/netrender/utils.py

@@ -245,8 +245,9 @@ def thumbnail(filename):
             process = subprocess.Popen(["convert", thumbname, "-resize", "300x300", thumbname])
             process.wait()
             return thumbname
-        except:
-            pass
+        except Exception as exp:
+            print("Error while generating thumbnail")
+            print(exp)
 
     return None