question: public final AssertSubscriber < T > assertErrorWith ( Consumer < ? super Throwable > expectation ) { assertNotComplete ( ) ; int s = errors . size ( ) ; if ( s == 0 ) { throw new AssertionError ( "No error" , null ) ; } if ( s == 1 ) { expectation . accept ( errors . get ( 0 ) ) ; } if ( s > 1 ) { throw new AssertionError ( "Multiple errors: " + s , null ) ; } return this ; }
target:  Assert an error signal has been received.