restore: Make missing messages non-fatal (again)

restore: Make missing messages non-fatal (again)

Previously, restore would abort if a message ID in the dump was
missing.  Furthermore, it would only report this as a warning.  This
patch makes it distinguish abort-worthy lookup failures like
out-of-memory from non-fatal failure to find a message ID.  The former
is reported as an error and causes restore to abort, while the latter
is reported as a warning and does not cause an abort.

This restores 0.14's non-fatal handling of missing message IDs in
restore (though 0.14 also considered serious errors non-fatal; we
retain the new and better handling of serious errors).