Merge pull request #920 from diffblue/ic3-fix

fixup! IC3: better error message when given assumption

Author: kroening

regression/ebmc/ic3/not_supported3.desc

@@ -2,6 +2,6 @@ CORE
 not_supported3.sv
 --ic3
 ^no support for assumptions$
-^EXIT=1$
+^EXIT=10$
 ^SIGNAL=0$
 --

src/ic3/m1ain.cc

@@ -111,7 +111,7 @@ property_checker_resultt ic3_enginet::operator()()
       if(property.is_assumed())
       {
         message.error() << "no support for assumptions" << messaget::eom;
-        return 1;
+        return property_checker_resultt::error();
       }
     }