fix error when sending thumbnail to master

Fix for #26867

Hunted down and debugged with great help from Geoff Murphy

* Scan for master failed and gave an script error. 

Based on patch [#28108] by Wintch Analyzer

Reported by Tobias Kummer. Thanks! :) 

by Wintch Analyzer 

moving master working path initialization before connection creation

also added error checks and prints to debug an invalid internal state (related to another tracker entry)

exception safeguard in render engine
fix module import broken by someone else

- remove/comment unused variables
- remove unused imports
- fixed some bugs using incorrect variables

fix problem on web interface with blacklisted slaves no longer in slave list

better stream handling when uploading files
Might solve the problem with OS X masters

Reported by Ramon Nieto

Added a better error message when the current file is not saved.

Add rna callback to server address property so that it correctly checks if the server exists when the value changes.

Report by Wintch Analyzer

Refill the rna blacklist list every refresh (safer that way).