timeout_util: Add error_message argument to Timeout()
timeout_util: Add error_message argument to Timeout()

This patch adds an optional argument to the Timeout context manager to
override the default "Timeout occurred" string that is put in the
TimeoutError exception when a timeout is hit. This is useful in cases
where you would want to rethrow an exception on timeout anyway, but with
a more descriptive message that can be caught and displayed by a
global exception handler or by Python itself.

BUG=None
TEST=Manual test with both default and custom message from CLI.

Change-Id: If1010ba7042d115c72757aa2138e2d3a545f991a
Signed-off-by: Julius Werner <4d12f06cff3725783b352157673a720bc67c4afe@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/205698
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite