Cover case when no device has been found.

README.rst

@@ -2,6 +2,9 @@
 Linak-ctrl
 ==========
 
+.. image:: https://badge.fury.io/py/linak-ctrl.svg
+   :target: https://badge.fury.io/py/linak-ctrl
+
 Simple python script to control Linak powered desks and USB2LIN06 cable.
 
 
@@ -14,6 +17,31 @@ Requirements
 * `pyusb`_
 
 
+Installation
+============
+
+There are couple of different ways for installing ``linak-ctrl``. One of the
+preferred ways is to use virtualenv and pip:
+
+.. code:: shell-session
+
+   $ git clone https://github.com/gryf/linak-ctrl
+   $ cd linak-ctrl
+   linak-ctrl $ python -m venv linak
+   (linak) linak-ctrl $ pip install .
+   (linak) linak-ctrl $ linak-ctrl status
+   Position: 767, height: 78.80cm, moving: False
+
+Or, you can install it system-wide:
+
+.. code:: shell-session
+
+   $ sudo pip install linak-ctrl
+
+And finally, you could also install dependences from your system repositories,
+and use script directly, by placing it somewhere in your ``$PATH``.
+
+
 Usage
 =====
 

linak_ctrl.py

@@ -111,6 +111,10 @@ class LinakDevice:
     def __init__(self):
         self._dev = usb.core.find(idVendor=LinakDevice.VEND,
                                   idProduct=LinakDevice.PROD)
+        if not self._dev:
+            raise ValueError(f'Device {LinakDevice.VEND}:'
+                             f'{LinakDevice.PROD:04d} '
+                             f'not found!')
 
         # detach kernel driver, if attached
         if self._dev.is_kernel_driver_active(0):
@@ -188,7 +192,11 @@ class LinakDevice:
 
 
 def main():
-    device = LinakDevice()
+    try:
+        device = LinakDevice()
+    except ValueError as ex:
+        sys.stderr.write(ex.args[0] + '\n')
+        sys.exit(1)
 
     parser = argparse.ArgumentParser('An utility to interact with USB2LIN06 '
                                      'device.')