buildbot: Add more debugging.

This adds the error returned from exec to the log message, and
provides '(empty)' placeholders when no stdout or stderr is present
from the called binary.

This has the effect of changing the silent errors I was seeing on my
buildbot from:

"""
Error:
	:
"""

To:

"""
Error:

	exec: "devcam": executable file not found in $PATH

Stdout:
(empty)

Stderr:
(empty)
"""

Also provides a /stderr handler linked from the status page that
allows all the logs to be viewed.  This is handy when the logs from an
individual test don't provide enough info to debug a build problem.

Change-Id: I4b3ef5a5db6aac1ddf41c41bc2fff310f19a82a1
Merge "buildbot: Add more debugging."
schema: fix a race on errors in FileReader.GetChunkOffsets, add tests

Change-Id: I51635d01c3c88c5f97ac15172c5b565f07dde350
Merge branch 'master' of https://camlistore.googlesource.com/camlistore