Add tup support

I am using [tup](https://github.com/gittup/tup) as a build system for some projects. I wanted to use rtags for these projects too. Since tup doesn't generate a compilation database automagically, I tried to use the compiler wrapper for this purpose. The problem is that tup uses a virtual filesystem to keep track of what is being done during compilation. Because of this, rtags reports wrong paths for the source files.

This modification aims to solve this problem. For this I had to modify the wrapper script. I created a filter to the path before running the rtags client so that the sources are searched for in the right directory.

While trying to find out the problems with the tup interworking, I also found a little bug in the code. When I cannot connect to the server, the error message is printed but the return code is still 0. I included the solution of this bug too.