add a type to the invalid offset error

lib/hermann/consumer.rb

@@ -1,4 +1,5 @@
 require 'hermann'
+require 'hermann/errors'
 
 if Hermann.jruby?
   require 'hermann/provider/java_simple_consumer'
@@ -26,7 +27,7 @@ module Hermann
       @topic = topic
 
       offset = opts.delete(:offset)
-      raise "Bad offset: #{offset}" unless valid_offset?(offset)
+      raise Hermann::Errors::InvalidOffsetError.new("Bad offset: #{offset}") unless valid_offset?(offset)
 
       if Hermann.jruby?
         zookeepers, group_id = require_values_at(opts, :zookeepers, :group_id)

lib/hermann/errors.rb

@@ -24,6 +24,9 @@ module Hermann
 
     # cannot discover brokers from zookeeper
     class NoBrokersError < GeneralError; end
+
+    # offsets can only be two symbols or a fixnum
+    class InvalidOffsetError < GeneralError; end
   end
 end