Inconsistent error message when removing tasks/events

[nus-se-pr-bot]

As a CS Student, 0 is a positive integer. However feel free to ignore this if it is expected behavior
Expected: Same error message when removing an event that does not exist.

Actual: Error message when index is greater than number of events is

And the error message when index is 0 is

_{[original: nus-cs2103-AY1718S2/pe-round1#119]}

Issue created by: @davidten

[Kyomian]

0 is NOT a positive integer; it is a non-negative integer, but I get your point Mr David