core: silence libunwind errors

Every libunwind error during backtrace collection is reported with
`say_error`. Since commit 19abfd2a ("misc: get rid of fiber_gc")
backtraces are collected on each fiber gc allocation, of which there are
plenty.

For some reason (https://github.com/tarantool/tarantool/issues/7980)
each unw_step fails on mac, and an error is spammed to instance logs,
even though the backtrace is actually collected.

Silence the errors, since there is no much use for them anyway. And
silence all of them just to be consistent.

This doesn't close #7980, because that issue still needs a proper fix.
Although its severity is ameliorated now.

In-scope-of #7980

NO_DOC=bugfix
NO_CHANGELOG=bugfix
NO_TEST=nothing to test