pw_env_setup: exit bootstrap.sh if not sourced
pw_env_setup: exit bootstrap.sh if not sourced

Exit bootstrap.sh (or activate.sh) if not sourced.

Bug: 169
Change-Id: I85954742f07eaec462354aee7685b142b0774b29

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed