multigit: set -e

Whenever a command fail, we want to abort the multigit script. That
would have let us detect that the script could not fetch patches from
Zuul (ZUUL_URL was pointing to wrong URL).

Impacts jobs:

beta-parsoid-update
parsoidsvc-parse-tool-check
parsoidsvc-parsertests-run-harder
parsoidsvc-roundtrip-test-check
visualeditor-doitall

Change-Id: I6eb61315a311f2fad18ced58b43af84fb52fcffd