task_wdt: fix silent init failures

The task_wdt_init() API can fail to install a timeout for the fallback
hardware WDT (hw_wdt) without returning an error code.  This patch
enables task_wdt_init() to return the hw_wdt install timeout error code
if the hw_wdt install timeout fails.

Signed-off-by: Nick Ward <nick.ward@setec.com.au>