Improve hypothesis output

[Stranger6667]

At the moment we have errors during CLI run in the following form:

Falsifying example: single_test(case=Case(path='/api/v2/users, method='PUT', path_parameters={}, headers={}, cookies={}, query={}, body={}, form_data={}), session=<requests.sessions.Session at 0x7f0370cb9510>, url='http://127.0.0.1:8080', checks=(<function schemathesis.runner.not_a_server_error>,), stats=StatsCollector(data=defaultdict(<class 'collections.Counter'>, {'not_a_server_error': Counter({'total': 2549, 'ok': 1875, 'error': 674})})))

Which contains some not relevant information which affects readability. Probably we can write it in a different, more readable way. Not that useful parts:

• test name. single_test is always there
• case keyword, the keyword itself doesn't bring much, its value is much more important
• stats, we will see them in the end and after implementing Display progress during the CLI run #125 they will be available during the CLI run
• requests session.
• checks representation. only the name is useful
  If we will remove those parts, then the output could look like this:

Falsifying example: 
  Parameters:
    - path='http://127.0.0.1:8080/api/v2/users'
    - method='PUT'
    - path_parameters={}
    - headers={}
    - cookies={}
    - query={}, 
    - body={}
    - form_data={}
  Failed checks: 
    - not_a_server_error

Also if we will not show empty case attributes, then the output will be even more readable

[Stranger6667]

Done via #193