Check server responses

Server responses are not checked properly right now. It all works well when the server works well, it can break when there's problem on the server side however.

Example of such a problem can now result in the following: (instead of a proper error message)

root@turris:~# netmetr
Checking uuid on the control server...
Traceback (most recent call last):
  File "/usr/bin/netmetr", line 11, in <module>
    load_entry_point('netmetr==1.5.5', 'console_scripts', 'netmetr')()
  File "/usr/lib/python3.7/site-packages/netmetr/__main__.py", line 622, in main
  File "/usr/lib/python3.7/site-packages/netmetr/__main__.py", line 121, in load_uuid
  File "/usr/lib/python3.7/site-packages/netmetr/__main__.py", line 142, in download_uuid
KeyError: 'settings'

(see https://gitlab.labs.nic.cz/turris/turris-os-packages/-/merge_requests/373#note_157445)