Throw appropriate error for pad_sequence input of length > maxlen

https://github.com/JuliaText/TextAnalysis.jl/blob/f11877268ba3911b30f7b13c549411a08cd380ad/src/sentiment.jl#L4

This function returns `nothing` in case the input length > maxlen.
It will be better to have a proper error handling and err message display for this.