finish Book rspecs

spec/gnucash/book_spec.rb

@@ -1,5 +1,19 @@
 module Gnucash
   describe Book do
+    context "with errors" do
+      it "raises an error for unexpected XML" do
+        gr = "gr"
+        Zlib::GzipReader.should_receive(:open).and_return(gr)
+        gr.should_receive(:read).and_return(nil)
+        ng = "ng"
+        Nokogiri.should_receive(:XML).and_return(ng)
+        ng.should_receive(:xpath).with('/gnc-v2/gnc:book').and_return([])
+
+        expect { Gnucash::Book.new('file name') }.to raise_error "Error: Expected to find one gnc:book entry"
+      end
+    end
+
+    context "without errors" do
       before(:all) do
         # just open the test file once
         @subject = Gnucash.open("spec/books/sample.gnucash")
@@ -22,3 +36,4 @@ module Gnucash
       end
     end
   end
+end