Crash if list command returns empty set of patches

[kvalo]

If there are no patches available pwcli crashes:

test-branch@pwcli-stub-Ta651k > 
potku:~/personal/patchwork/src/pwcli$ ./run_stub 
Connecting to http://localhost:8106/
User          : test (7477, 30052, 118371)
Projects      : stub-test
Tree          : pwcli-stub-Hsbz4D
Branch        : test-branch
New           : 18
Review        : 7
Upstream      : 0
Deferred      : 5
Total         : 30
test-branch@pwcli-stub-Hsbz4D > list -n doesnotexist

test-branch@pwcli-stub-Hsbz4D > review 2
Traceback (most recent call last):
  File "/home/kvalo/projects/personal/patchwork/src/pwcli/pwcli", line 2217, in <module>
    main()
  File "/home/kvalo/projects/personal/patchwork/src/pwcli/pwcli", line 2214, in main
    pwcli.run_shell()
  File "/home/kvalo/projects/personal/patchwork/src/pwcli/pwcli", line 2002, in run_shell
    args.func(args)
  File "/home/kvalo/projects/personal/patchwork/src/pwcli/pwcli", line 1664, in cmd_review
    patches = self.get_patches_from_ids(args.ids)
  File "/home/kvalo/projects/personal/patchwork/src/pwcli/pwcli", line 958, in get_patches_from_ids
    raise Exception('No patches available, doing nothing.')
Exception: No patches available, doing nothing.