How about raise ArgumentError rather than just calling error() in insertcols!()?

[petershintech]

I found the following case is the perfect case to raise ArgumentError(). What do you think of it?

function insertcols!(df::DataFrame, col_ind::Int, name_col::Pair{Symbol, <:AbstractVector};
                     makeunique::Bool=false)
    name, item = name_col
    0 < col_ind <= ncol(df) + 1 || throw(BoundsError())
    size(df, 1) == length(item) || size(df, 2) == 0 || error("number of rows does not match")

    if hasproperty(df, name)

[bkamins]

Sure - feel free to open a PR to improve both throw(BoundsError()) and error.

[bkamins]

fixed