fs_mgr: error check umount calls / add retry logic

Don't silently ignore umount errors. At a minimum, log them.

Add strerror(errno) to another umount call, to make sure we get
some actionable data.

check_fs: try 5 times umounting the filesystem. It appears that
the umount is failing, perhaps because some service is opening
the file on the device or other error. Try unmounting it multiple
times in case it's a transient problem and we can recover.

Bug: 19199624
Bug: 19156134
Change-Id: I7213eb52d55116fb2419a36494d26d5e159981a7