FIX run() should use the same python as the install/etc.
(cherry picked from commit 3cd981e91fca49aa13bae30d8c3e4470e20a0388)
BUG: handle proxy string config without http://.

Not sure this is 'legal', but let's support it for now.
BUG: reject invalid proxy string, and closes #146.
MAINT: forgot to commit enstaller.errors.
BUG: don't crash with an UnboundLocalError for invalid pinfo.
BUG: fix setup_proxy_info to handle empty string.
REF: move AuthFailedError into enstaller.errors.