fix some sync issues

- better error message when upload fails
- don't show success message on error
- fix 'unable to find server' accidentally being catchall
fix "free variable 'mw'" error (#284)
add link to add-on forum in error
Merge branch 'master' of github.com:dae/anki
Merge branch 'master' of github.com:dae/anki
when given base on cmdline, get abspath