Extract the error message from the shell script using Python?

I have a simple Python script which will execute a shell script using subprocess mdoule in Python.

Below is my Python shell script which is calling testing.sh shell script and it works fine.

import os
import json
import subprocess

jsonData = '{"pp": [0,3,5,7,9], "sp": [1,2,4,6,8]}'
jj = json.loads(jsonData)

print jj['pp']
print jj['sp']

os.putenv( 'jj1',  'Hello World 1')
os.putenv( 'jj2',  'Hello World 2')
os.putenv( 'jj3', ' '.join( str(v) for v in jj['pp']  ) )
os.putenv( 'jj4', ' '.join( str(v) for v in jj['sp']  ) )

print "start"
proc = subprocess.Popen('testing.sh', stdout=subprocess.PIPE, stderr=subprocess.PIPE)
(stdout, stderr) = proc.communicate()
if stderr:
   print "Shell script gave some error"
else:
   print "end" # Shell script ran fine.

And below is my testing.sh shell script -

#!/bin/bash

for el1 in $jj3
do
    echo "$el1"
done

for el2 in $jj4
do
    echo "$el2"
done

for i in $( david ); do
    echo item: $i
done

Now the question I have is -

If you see my Python script, I am printing out start and then executing my shell script, if there are any errors in executing my shell script, then it won't print out end. In the above example, it won't print out end as there is an error.

1. Firstly, So is this the right way to do this kind of problem? Basically, I want to print start first and if the shell script got executed successfully, then I want to print end. But if for whatever reason, my shell script doesn't got executed successfully, then I won't print out end.
2. Secondly, with this approach, I cannot see my echo statement getting printed out on the console from the shell script? Does it mean, my shell script are getting executed but it is not getting printed out on the console somehow?
3. Thirdly, Is there any possibility of extracting the error message from the shell script, meaning whatever was the reason of shell script getting failed?