make all catch() arguments const

- I saw this on http://en.cppreference.com/w/cpp/language/try_catch and
  thought it would be a good idea
- also unify used format to better be able to search for exception
  uses in our codebase

src/test/alert_tests.cpp

@@ -92,7 +92,7 @@ struct ReadAlerts
                 alerts.push_back(alert);
             }
         }
-        catch (std::exception) { }
+        catch (const std::exception&) { }
     }
     ~ReadAlerts() { }