RF: move exceptions used by both client and server into common/exceptions.py

this prevents importing of server while operating with client only
BF: do not rely on scripts being under /usr -- might differ eg on Fedora -- rely on import of common.version (Closes gh-112)

This is also not ideal, since if there happens to be some systemwide common.version -- we are doomed

but otherwise, we cannot keep extending comparison check to /bin, /sbin whatelse