Fix various build script errors

[mtneug]

This fixes various small errors in scripts of the build image:

• remove set -e were a failure should not stop the script
• replace falsely used return with exit
• cleaning of directories has not matched any file before (nothing was removed)
• during install some folders were not yet created

To verify run these commands from the root of this repository:

$ make build-build

$ export OPENCAST_SRC=$(mktemp -d)
$ docker-compose -p opencast-build -f docker-compose/docker-compose.build.yml up -d
$ docker-compose -p opencast-build -f docker-compose/docker-compose.build.yml exec opencast bash

$ oc_clone
# This should clone the Opencast repo and check out 2.3.0
$ oc_build
# This should build Opencast
$ oc_install allinone
# This should install the allinone distribution; it is extracted into /opencast
$ oc_run
# This should start Opencast; exit with ctrl+c
$ oc_uninstall
# This should uninstall Opencast, i.e. the directory /opencast is empty
$ oc_clean_data
# This should remove user data, i.e. the directory /data is empty
$ exit

$ docker-compose -p opencast-build -f docker-compose/docker-compose.build.yml down -v
$ sudo rm -rf $OPENCAST_SRC

The three is_* scripts are checked implicitly as they are called from other scripts.