of a (NOR) flash chip: allow passing "last" as an alias
for the number of the last sector.

Improve several aspects of error checking while we're at it.

From: Johnny Halfmoon <jhalfmoon@milksnot.com>


git-svn-id: svn://svn.berlios.de/openocd/trunk@2746 b42882b7-edfa-0310-969c-e2dbd0fdcd60