Nested IF conditions are not supported in Personas 2.0. The way to achieve something close to that is having an IF condition which will push another script button that has the second IF condition and so on. Can become a little convoluted if you have many conditions 
As for capturing the message from the status bar, that should be no problem with the field ID you mentioned. Can you not copy that to a variable? If you did that, then you only have to navigate back to your previous screen and paste the error text into your target field.