[PE-D][Tester D] Error message for index faulty

[nus-pe-bot]

when a index over the number of students in the list is used, a relevant error message that the index is invalid is shown

but

when an index below 0 is used, the error message that is shown is invalid command format
which is untrue as the command format is correct, its just the index that is invalid

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Labels: type.FunctionalityBug severity.Medium
original: wapisai/ped#11