Bad error messages in parsing of assignment/swap

[mbudde]

When there's an error in the left-hand side of an assignment or swap statement the error is a little confusing. E.g.

procedure main()
    x[#] += 5

give the error

File "tests/errors/parse-error-assign.ja" in line 2, column 7:
    Unexpected "#"
    Expecting statement

The problem is that the left-values are parsed wrapped in try so when they fail, parsing of the whole statement fails.