Down-level log message for errors that can be handled

We deliberately have many functions that do not abort the process
on failure and instead simply return a corresponding value. It
is then up to the caller to decide how to handle this failure;
in particular, such a failure can be expected, e.g., if we try to
fetch a file from remote execution first, before fetching it from
the upstream location.

To have a consistent user experience, nothing that can occur in
a successfull build should be reported at error level; moreover,
messages that routinely occur during successfull builds should not
be reported at progress or above, except for the (stage) result
messages and the progress reporter.

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