Fix error messages

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Files with missing lines	Coverage Δ	Complexity Δ
.../seedu/address/logic/commands/AddBuyerCommand.java	85.18% <100.00%> (+0.56%)	8.00 <0.00> (ø)
...seedu/address/logic/commands/AddClientCommand.java	62.50% <ø> (-4.17%)	2.00 <0.00> (ø)
...seedu/address/logic/commands/AddSellerCommand.java	85.71% <100.00%> (+0.52%)	8.00 <0.00> (ø)
...edu/address/logic/commands/DeleteBuyerCommand.java	81.48% <100.00%> (ø)	7.00 <0.00> (ø)
...du/address/logic/commands/DeleteClientCommand.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
...du/address/logic/commands/DeleteSellerCommand.java	80.76% <100.00%> (ø)	7.00 <0.00> (ø)