Fix stack size check on systems with less than 256kb stack

We require at least 256 kbytes slack stack space, and if a system is
configured with less (or equal), then the stack size checks fail early on
and strange error message occur during engine startup and execution.

This patch calls the stack check code early on and bails out with an error
message that's more descriptive.

Change-Id: I3263f2f93f62332d08003411e1bb5b3b1140d02b
Task-number: QTBUG-41213
Reviewed-by: Lars Knoll <lars.knoll@digia.com>