Logging errors with the debug format specifier causes multi-line errors,
which are sometimes a pain to deal with. Instead, we should use anyhow's
alternate display format, which shows the same information on a single
line.
Also adjusted a couple of error messages that were stale.
Fixesneondatabase/cloud#14710.
Part 1 of 2, for moving the file cache onto disk.
Because VMs are created by the control plane (and that's where the
filesystem for the file cache is defined), we can't rely on any kind of
synchronization between releases, so the change needs to be
feature-gated (kind of), with the default remaining the same for now.
See also: neondatabase/cloud#6593
