def getTestRunError(connection, sessionUrl):
    '''
    This method gets the error that appeared during the last test run.
    If no error appeared (the test ran successfully), the return value will be 'None'.
    Args:
        - connection is the connection object that manages the HTTP data transfers between the client and the REST API
        - sessionUrl is the address of the session that should run the test.
    '''
    activeTestUrl = "%s/ixload/test/activeTest" % (sessionUrl)
    testObj = connection.httpGet(activeTestUrl)

    return testObj.testRunError