view_builder: do_build_step: handle unexpected exceptions

Exception are handled by do_build_step in principle,
Yet if an unhandled exception escapes handling
(e.g. get_units(_sem, 1) fails on a broken semaphore)
we should warn about it since the _build_step.trigger() calls
do no handle exceptions.

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>

db/view/view.cc

@@ -1844,6 +1844,8 @@ future<> view_builder::do_build_step() {
                 _current_step = _base_to_build_step.begin();
             }
         }
+    }).handle_exception([] (std::exception_ptr ex) {
+        vlogger.warn("Unexcepted error executing build step: {}. Ignored.", std::current_exception());
     });
 }