notmuch-new: Check for non-fatal errors from stat()

Currently we assume that all errors on stat() a dname is fatal (but
continue anyway and report the error at the end). However, some errors
reported by stat() such as a missing file or insufficient privilege,
we can simply ignore and skip the file. For the others, such as a fault
(unlikely!) or out-of-memory, we handle like the other fatal errors by
jumping to the end.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>