Documentation: move the error handling to the better place in DMA-API-HOWTO

Handing DMA mapping errors is essential.  Let's put it in the more
appropriate place rather than the end of the doc.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>