Fix errors with invalid csv

[BigG1947]

We have 2 situation when raised errors without handling:

1. Empty CSV and model has force_encoding: 'auto'
   In this case encoding detected automatically by

 def dynamic_encoding(data) CharDet.detect(data)['encoding'] end

But if data is empty string '' result will be nil. data.force_encoding(encoding_name) where data is empty string and encoding_name is nil raise TypeError: no implicit conversion of nil into String

2. When we have invalid csv in which values exceeded headers activerecord_import gem raised ArgumentError:

 raise ArgumentError, "Number of values (#{arr.length}) exceeds number of columns (#{columns.length})"

We can handle it in collection_action and return flash message to users