rss2i.py: restore: KeyError when no session data is provided

A KeyError occurs if a restore for a device x is requested but there is no session data for that device x in the provided session data file. Note, that such a situation may occur due to #22 (closed).

Reproduce

Assume you have devices x and y. Device y is connected via USB. Perform some measurement with both devices. Make sure device x is turned off afterwards. Then record the session data:

rss2i -a record -d x,y

An OTA timeout will occur that isn't handled properly (#22 (closed)):

rss2i: Detecting connected devices... [y]
rss2i: Device y --over-the-air--> device x
rss2i: _query@2125: WARN_OVERTHEAIR_TIMEOUT: timeout ocurred when trying to send data to device x
rss2i: Disconnecting devices...ok

Afterwards, the session data of device y are stored in rec.json. The ones of device x are not. Now, try to restore data from both devices using the recorded session data:

rss2i -a restore -f config.txt -d x,y -r rec.json

rss2i: Detecting connected devices... [y]
rss2i: Disconnecting devices...ok
Traceback (most recent call last):
  File "/home/maus/bin/rss2i", line 2826, in <module>
    main()
  File "/home/maus/bin/rss2i", line 2801, in main
    args.func(args)
  File "/home/maus/bin/rss2i", line 1041, in restore
    address = records[str(devID)]['Address']
KeyError: 'x'

Desired Behaviour

That KeyError should be catched in some way.