Add `set -e` and wrap whole section in a single "scl enable"

[irushchyshyn]

• Reverted and reworked 9a02d4d as it introduced regression. Added more tests.
• Fix issue #17:
  • Do set -e so that the script behaviour is consistent regardless of whether the script is executed in the rpm shell or the scl environment. https://bugzilla.redhat.com/show_bug.cgi?id=1184515 https://www.softwarecollections.org/en/docs/guide/#sect-Converting_RPM_Scripts
  • Wrap whole section in a single "scl enable"