DEV: Stop if theme:update fails for default site (PR #15090)

The error handling of the theme:update Rake task has been improved. If an error occurs while updating the default site, then the exception will be propagated and the process will exit with non-zero status.

This is a follow-up to commit 3f97f884fef9d3e4b33ae6361d7891c3c414802b.


this addition doesn’t seem to be necessary? :slightly_smiling_face: