Made error output cleaner

lib/apex/frame/immutable_path.rb

@@ -14,7 +14,7 @@ module Apex
       hops.each do |hop|
         raise ArgumentError.new("All arguments must not be nil: #{hops.to_s}") if (hop.nil?)
         raise ArgumentError.new("All arguments must be of type Hop: #{hops.class.to_s}") if not hop.kind_of? Apex::Hop
-        raise ArgumentError.new("A seen hop can not follow an unseen hop: #{hops.to_s}") if (hop.seen?) && (!last_seen)
+        raise ArgumentError.new("A seen hop can not follow an unseen hop: #{hops.join(',').to_s}") if (hop.seen?) && (!last_seen)
         last_seen = false if not hop.seen?
         @path_array << hop
       end