shared/install: fix bogus error handling

shared/install: fix bogus error handling

This partially reverts a4f0e0da3573a10bc5404142be8799418760b1d1. The
intent was good, we gather the errors, but we have no mechanism to
propagate the result, so the gathered result was ignored. In 'changes'
we can only report errors for specific units. If reading of the
directory fails, we might just as well report the error immediately.
This isn't great, but it's better then ignoring the errors. In practice,
failing halfway in this manner is unlikely, since it'd mean that the fs
is corrupted or something like that. We might as well return immediately
on such catastrophic errors.