Continue after failed assertion

1- Double click on the test case (not test step)

look at the provided picture and find the one shown below:

In Ready Api (Soapui pro) right click on Test Case and click on Option.

Abort test if an error occurs (Disable)

To continue tests executing after failed test step you need disable "Fail on error" option in TestCase options.

To control test execution flow you can use "Conditional Goto" test step (the easiest way) or "Groovy script" test step if you need more complex logic.

Detailed instructions here: http://www.soapui.org/Functional-Testing/controlling-flow.html