upload: Catch and cleanly report connectivity errors

Instead of giving a Python backtrace when there is a connectivity
problem during repo upload, report that we cannot access the host,
and why, with a halfway decent error message.

Bug: REPO-45
Change-Id: I9a45b387e86e48073a2d99bd6d594c1a7d6d99d4
Signed-off-by: Shawn O. Pearce <sop@google.com>