Add more error checking to BuildSpecsManager setup.
Add more error checking to BuildSpecsManager setup.

1. RunCommand may throw errors where error is set to None.
   We should check for this.
2. self.latest may be set to None if there are no spec files
   in the directory. We should check for this.
3. GetBuildStatus should not actually run gsutil if we're in dry_run mode,
   because the user may not have gsutil installed.

BUG=none
TEST=Run x86-mario-release with no gsutil in /b

Change-Id: I8bb196503da8f88dbc7c1a3c441c4b938e9754db
Reviewed-on: https://gerrit.chromium.org/gerrit/29837
Commit-Ready: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Reviewed-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>

zhang0137/chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,coreos/chromite,coreos/chromite,zhang0137/chromite,chadversary/chromiumos.chromite,coreos/chromite,chadversary/chromiumos.chromite