Improve logs msg for command related exception

Logger will log "Invaid Command: <command text>" when a CommandException
or ParseException occur.

However, the exceptions might be caused by other issuess like unable to
write to save file etc.
Therefore, the current log message is not suitable for CommandException.

Let's improve the accuracy of the error message by changing it to
accurately describe both CommandException and ParseException.

src/main/java/seedu/address/ui/MainWindow.java

@@ -188,7 +188,7 @@ private CommandResult executeCommand(String commandText) throws CommandException
 
             return commandResult;
         } catch (CommandException | ParseException e) {
-            logger.info("Invalid command: " + commandText);
+            logger.info("An error occurred while executing command: " + commandText);
             resultDisplay.setFeedbackToUser(e.getMessage());
             throw e;
         }