Cocoa: Clean up context creation.

Make the fallback to creating an unshared context
actually work by using correct [initWithFormat: ]
code.

Warn and return early on context creation failure
instead of continuing and crashing.

Change-Id: Ic88f419eaa717436aefc9c1da36c47e0ccb3e956
Task-number: QTBUG-47825
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>