Include the failed directory name if _mkdir fails.

http://b.android.com/202972 shows a failure here, but we lack the
information to know what the bad input was. My guess is that it's
caused by POSIX path separators, but we should add more details to
the error message for the future.

Bug: http://b.android.com/202972
Change-Id: I29782aa6352da5e953055acdee0ae11f54484685
1 file changed