Crash when no provider is present

[Maciej Kisielewski]

Checkbox Converged crashes when there are no providers installed.

"PyOtherSide error: Traceback (most recent call last):

  File "/home/kissiel/checkbox/checkbox-touch/py/checkbox_touch.py", line 154, in __init__
    additional_providers=self._get_embedded_providers(providers_dir))

  File "/home/kissiel/checkbox/plainbox/plainbox/impl/decorators.py", line 104, in wrapper
    raise exc

  File "/home/kissiel/checkbox/plainbox/plainbox/impl/decorators.py", line 98, in wrapper
    return func(*args, **kwargs)

  File "/home/kissiel/checkbox/plainbox/plainbox/impl/session/assistant.py", line 303, in select_providers
    raise ValueError("nothing selected with: {}".format(pat))

ValueError: nothing selected with: *