Fixed BUG-18 -> cannot find book

BUGs

@@ -1,5 +1,4 @@
-#### BUGS (next-18)
-BUG-18:		delete a book, then go to that URL and you get odd DB errors, rather than book not found
+#### BUGS (next-19)
 
 ### DB/back-end
 BUG-2:		series does not deal with calcd_rating... 

main.py

@@ -593,6 +593,11 @@ def book(id):
             st.SetMessage(message)
     else:
         book = Book.query.get(id)
+        if book == None:
+            st.SetAlert("danger")
+            st.SetMessage("Cannot find Book (id={})".format(id))
+            return render_template("base.html", alert=st.GetAlert(), message=st.GetMessage())
+
         book_form=BookForm(obj=book)
 
     author_list = GetAuthors()