dir-iterator: use warning_errno when possible

dir-iterator: use warning_errno when possible

Change warning(..., strerror(errno)) by warning_errno(...). This helps
to unify warning display besides simplifying a bit the code. Also,
improve warning messages by surrounding paths with quotation marks and
using more meaningful statements.

Signed-off-by: Matheus Tavares <redacted>
Signed-off-by: Junio C Hamano <redacted>