Unclear error message for `delete` command for negative indices/ without starting index input

[tingxuanp]

To recreate:

delete -2

and separately,

delete-0

The error message for the delete -2 command, which is either one of the two cases:

1. when the user specifies an invalid index (negative number)
2. When the user wants to delete contacts from a range of indexes without specifying the starting index.

The error message shows that the application seems to treat this as a case where the starting index is not smaller than or equal to the end index. But this is not specific enough because the starting index is not even specified. I think this error message can be improved. If the team wishes to treat the delete -2 command as an invalid input because the starting index has not been specified, I think it can be stated more clearly.

For example, the error message could have been "The starting index should be specified".

[nus-se-script]

Your response not required for this bug as the team has accepted the bug as it is.