Fix gcc warning: use const references instead of pass by value for exceptions.

src/common/JoyMap.cxx

@@ -240,7 +240,7 @@ int JoyMap::loadMapping(const json& eventMappings, const EventMode mode)
       );
 
       i++;
-    } catch (json::exception) {
+    } catch (const json::exception&) {
       Logger::error("ignoring invalid joystick event");
     }
   }

src/common/KeyMap.cxx

@@ -208,7 +208,7 @@ int KeyMap::loadMapping(const json& mappings, const EventMode mode) {
       );
 
       i++;
-    } catch (json::exception) {
+    } catch (const json::exception&) {
       Logger::error("ignoring bad keyboard mapping");
     }
   }

src/common/PJoystickHandler.cxx

@@ -47,7 +47,7 @@ PhysicalJoystickHandler::PhysicalJoystickHandler(
 
   try {
     mappings = json::parse(serializedMapping);
-  } catch (json::exception) {
+  } catch (const json::exception&) {
     Logger::info("converting legacy joystick mappings");
 
     mappings = convertLegacyMapping(serializedMapping);

src/common/PKeyboardHandler.cxx

@@ -74,7 +74,7 @@ void PhysicalKeyboardHandler::loadSerializedMappings(const string& serializedMap
 
   try {
     mapping = json::parse(serializedMapping);
-  } catch (json::exception) {
+  } catch (const json::exception&) {
     Logger::info("converting legacy keyboard mappings");
 
     mapping = KeyMap::convertLegacyMapping(serializedMapping);
@@ -82,7 +82,7 @@ void PhysicalKeyboardHandler::loadSerializedMappings(const string& serializedMap
 
   try {
     myKeyMap.loadMapping(mapping, mode);
-  } catch (json::exception) {
+  } catch (const json::exception&) {
     Logger::error("ignoring bad keyboard mappings");
   }
 }

src/common/PhysicalJoystick.cxx

@@ -87,7 +87,7 @@ bool PhysicalJoystick::setMap(const json& map)
 
     try {
       joyMap.loadMapping(entry.value(), eventModeFromJsonName(entry.key()));
-    } catch (json::exception) {
+    } catch (const json::exception&) {
       Logger::error("ignoring invalid json mapping for " + entry.key());
     }