When an attempt to set watchpoint fails because there is no free triggers...

When an attempt to set watchpoint fails because there is no free triggers OpenOCD reports "unknown error"

Now it returns `resource not available`

Change-Id: Ifbbd468bdf62023850690eb96fe8a16f4114e915
Signed-off-by: Kirill Radkin <kirill.radkin@syntacore.com>