Only delete the build dir in tester recipes if it exists.
Only delete the build dir in tester recipes if it exists.

The 'rmdir build' step fails if the build dir does not exist, e.g. after a former extract build failure that already nukes the parent directory. Once this state is reached, the bot can't recover anymore.

Review URL: https://codereview.chromium.org/216633004

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@261152 0039d316-1c4b-4281-b951-d872f2087c98

luci/recipes-py,luci/recipes-py,shishkander/recipes-py,shishkander/recipes-py