Provide more detailed error message

If we don't have a mapping for a column name, provide the column name
as well as the index in the array.

lib/activerecord-import/import.rb

@@ -19,8 +19,8 @@ module ActiveRecord::Import #:nodoc:
   end
 
   class MissingColumnError < StandardError
-    def initialize(index)
+    def initialize(name, index)
-      super "Missing column for value at index #{index}"
+      super "Missing column for value <#{name}> at index #{index}"
     end
   end
 end
@@ -276,7 +276,13 @@ class ActiveRecord::Base
     # information on +column_names+, +array_of_attributes_ and
     # +options+.
     def import_without_validations_or_callbacks( column_names, array_of_attributes, options={} )
-      columns = column_names.map { |name| columns_hash[name.to_s] }
+      columns = column_names.each_with_index.map do |name, i|
+        column = columns_hash[name.to_s]
+
+        raise ActiveRecord::Import::MissingColumnError.new(name.to_s, i) if column.nil?
+
+        column
+      end
 
       columns_sql = "(#{column_names.map{|name| connection.quote_column_name(name) }.join(',')})"
       insert_sql = "INSERT #{options[:ignore] ? 'IGNORE ':''}INTO #{quoted_table_name} #{columns_sql} VALUES "
@@ -308,8 +314,6 @@ class ActiveRecord::Base
         my_values = arr.each_with_index.map do |val,j|
           column = columns[j]
 
-          raise ActiveRecord::Import::MissingColumnError.new(j) if column.nil?
-
           if val.nil? && !sequence_name.blank? && column.name == primary_key
              connection.next_value_for_sequence(sequence_name)
           else