Fix exception messages for logAdd command

Author: CJerrong

src/main/java/scrolls/elder/logic/commands/LogAddCommand.java

@@ -49,7 +49,7 @@ public class LogAddCommand extends Command {
         + PREFIX_REMARKS + "was a good session ";
 
     public static final String MESSAGE_SUCCESS = "New log added!";
-    public static final String MESSAGE_NEGATIVE_DURATION = "Duration cannot be negative.";
+    public static final String MESSAGE_NEGATIVE_DURATION = "Duration must be positive.";
     public static final String MESSAGE_PERSONS_NOT_PAIRED = "The volunteer and befriendee are not paired.";
 
     /**

src/main/java/scrolls/elder/logic/parser/ParserUtil.java

@@ -173,7 +173,7 @@ public static Integer parseInt(String num) throws ParseException {
         try {
             return Integer.parseInt(trimmedNum);
         } catch (NumberFormatException e) {
-            throw new ParseException("Invalid number format. Expected a number.", e);
+            throw new ParseException("Invalid number format. Expected an integer.", e);
         }
     }
 }