Raise the catched exception for failed upload.

Summary:
without this diff, the retry may not work properly.

Test Plan: manually test

Reviewers: lguo

CC:

Task ID: #

Blame Rev:
add a unit test for the dual uploader.
minor changes
add a wal-check function in wal-e to check if a segment exists on s3.

Summary:
as the title, check if single segment exists

Test Plan: run it.
envdir /etc/wal-e.d/env /usr/local/bin/wal-e wal-check pg_xlog/0000000600004C02000000B8

returncode is 0 if exists.

Reviewers: lguo

CC:

Task ID: #

Blame Rev:
Merge pull request #3 from DikangGu/v0.7.0insta2

Raise the catched exception for failed upload.