Fix #3401 warning on multiple solid

- PartDesign only uses the first result shape
  of an operation and discards the rest without
  warning.

- this also fixes #1707

src/Mod/PartDesign/App/FeatureBoolean.cpp

@@ -141,6 +141,11 @@ App::DocumentObjectExecReturn *Boolean::execute(void)
 
         result = boolOp; // Use result of this operation for fuse/cut of next body
     }
+
+    int solidCount = countSolids(result);
+    if (solidCount > 1) {
+        return new App::DocumentObjectExecReturn("Boolean: Result has multiple solids. Check parameters.");
+    }
 
     this->Shape.setValue(getSolid(result));
     return App::DocumentObject::StdReturn;