jjb: be verbose and fail early on errors

Prior to this commit, if the "job-builder update" command failed,
Jenkins would still consider the overall job to have succeeded, because
the jjb/config/config script was exiting with a zero exit code. (This
was because the for/ls loop were the final commands to run, and they
were exiting with zero.)

When I modified the Jenkins UID as a part of the overall Jenkins
authentication changes yesterday, the "job-builder update" command no
longer worked, because the password in jenkins_builder.ini was no longer
valid. However, Jenkins still considered the jjb job to be passing, as
explained above.

Tell bash to be verbose about the commands it runs ("set -x") and fail
early if there are any problems ("set -e").

With this change, if there is a problem with "job-builder update" or any
other command in the script, the job will fail right away and Jenkins
will bring it to our attention sooner.
Merge pull request #21 from ktdreyer/jjb-errors

jjb: be verbose and fail early on errors