User-friendly error if no platform/device was found

[LWisteria]

On a machine with no platform, current ClPy got an error as follows:

$ python
Python 3.6.5 (default, Feb 12 2019, 02:34:25)
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import clpy
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/username/.pyenv/versions/clpy/lib/python3.6/site-packages/clpy-2.1.0b1-py3.6-linux-x86_64.egg/clpy/__init__.py", line 17, in <module>
    from clpy import core  # NOQA
  File "/home/username/.pyenv/versions/clpy/lib/python3.6/site-packages/clpy-2.1.0b1-py3.6-linux-x86_64.egg/clpy/core/__init__.py", line 1, in <module>
    from clpy.core import core  # NOQA
  File "clpy/backend/device.pxd", line 4, in init clpy.core.core
  File "/home/username/.pyenv/versions/clpy/lib/python3.6/site-packages/clpy-2.1.0b1-py3.6-linux-x86_64.egg/clpy/backend/__init__.py", line 4, in <module>
    from clpy.backend import device  # NOQA
  File "clpy/backend/device.pyx", line 1, in init clpy.backend.device
  File "clpy/backend/opencl/env.pyx", line 81, in init clpy.backend.opencl.env
  File "clpy/backend/opencl/api.pyx", line 16, in clpy.backend.opencl.api.GetPlatformIDs
  File "clpy/backend/opencl/exceptions.pyx", line 23, in clpy.backend.opencl.exceptions.check_status
clpy.backend.opencl.exceptions.OpenCLRuntimeError: UNKNOWN ERROR: -1001

That's bad (what's the unknown error??). The correct exception must be raised.