Don't do a broken sync when the system exclude file is missing

I had never formally installed the client and so there was no exclude file on my system. That lead to, among others, the temporary download files being synced to the server.

Fixes:
1) Don't sync when there is no system exclude file. We would end up in a confusing, broken state.
2) Fall back to the exclude file next to the binary if there is no system exclude file to make life easier for developers.
3) Fix a typo in a invokeMethod() slot name

CC @dragotin @danimo @guruz @ogoffart 
Coding style:  @dragotin insist that we always have braces. 

(the brace on the same line as the if)