Adds extra measure of safety to dartc.sh and dartc_test.sh

Adds extra measure of safety to dartc.sh and dartc_test.sh

Adding set -e will cause the script to exit with non-zero status if a 
shell command exits with non-zero and the status is not checked.  We found
a case where a problem wasn't being caught and the script kept going in another
change.  This might help prevent similar problems from going uncaught in the
future.

Committed: https://code.google.com/p/dart/source/detail?r=3148

[zundel, 2012-01-10 15:19:30]

See also http://codereview.chromium.org/9158011/

I'm not sure this change would have caught that error though...

[codefu, 2012-01-10 15:26:05]

lgtm
I didn't know build bot was using /bin/dash.  Never heard of dash till this
morning!