bin: set error on exit for shell scripts (#7580)

I am building Roundcube via CI jobs, and also ran in #7567 (thank you
for the quick fix!).

The CI job I'm using runs css/js shrink and the build job itself
succeeded, although Roundcube wasn't working.

With this commit all shell scripts will propagate the exit code in case
of errors, e.g. failed downloads.