Bug/fix edit command index

[dominic2412]

close #125
Fix edit command to display format message when no index is given, and when only have index prompt the user to key in at least one change

when index is wrong, user are prompted to look at the list of valid candidates

when the index is correct but no field, another customised message comes out
"At least one field to edit must be provided."

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ	Complexity Δ
src/main/java/seedu/address/logic/Messages.java	91.66% <ø> (ø)	3.00 <0.00> (ø)
...java/seedu/address/logic/commands/EditCommand.java	97.27% <ø> (ø)	13.00 <0.00> (ø)
.../seedu/address/logic/parser/EditCommandParser.java	96.07% <100.00%> (-0.08%)	16.00 <0.00> (ø)
...in/java/seedu/address/logic/parser/ParserUtil.java	98.36% <ø> (ø)	24.00 <0.00> (ø)

... and 7 files with indirect coverage changes

[stanleytangzh]

LGTM