haskell tutorial: fix up division by 0 example

+ pushd .cabal-sandbox/bin
~/packages/thrift/.cabal-sandbox/bin ~/packages/thrift
+ trap 'kill -9 17219' EXIT
+ ./HaskellServer
+ ./HaskellClient
"Starting the server..."
HaskellClient: connect: does not exist (Connection refused)
+ sleep 0.1
+ ./HaskellClient
"ping()"
"ping()"
add(1,1)
1+1=2
calculate(1, Work {f_Work_num1 = Just 1, f_Work_num2 = Just 0, f_Work_op = Just DIVIDE, f_Work_comment = Nothing})
InvalidOperation InvalidOperation {f_InvalidOperation_what = Just 4, f_InvalidOperation_why = Just "Cannot divide by 0"}
calculate(1, Work {f_Work_num1 = Just 15, f_Work_num2 = Just 10, f_Work_op = Just SUBTRACT, f_Work_comment = Nothing})
15-10=5
Check log: 5
+ popd
~/packages/thrift
+ kill -9 17219