Do not use context manage to preserver Python 2.6 compatibility

Test `scc unrebased-prs` return code using a try/except block
merge master -D org -I include -E exclude: PR 106 (Add a new CheckStatus command to retrieve Github API status)
merge master -D org -I include -E exclude: PR 107 (Add additional choices in --check-commit-status)
merge master -D org -I include -E exclude: PR 108 (Improve scc unrebased-prs output)