Ensure upload of images actually succeeds

[mkesper]

Whether the upload is using curl as right now or the method proposed in #119, it should always check that there were no errors.
Right now, the result of the curl command isn't checked at all:
OvfManager.rb:
``

    system("#{downloadCmd} | #{uploadCmd}", :out => "/dev/null")
    
    keepAliveThread.kill
    keepAliveThread.join
    
    progress += (90.0 / result.fileItem.length)
    nfcLease.HttpNfcLeaseProgress(:percent => progress.to_i)
  end

  nfcLease.HttpNfcLeaseProgress(:percent => 100)
  raise nfcLease.error if nfcLease.state == "error"

It should also raise if the system command returns an error.

[mkesper]

Example:

2019-01-07 11:43:53 +0100: Uploading OVF to przlhyp711.example.com...
DEBUG: Timeout: 300
Iteration 1: Trying to get host's IP address ...
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed

  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
  0  740M    0 1100k    0     0  1051k      0  0:12:00  0:00:01  0:11:59 1051k
  0  740M    0 5900k    0     0  3243k      0  0:03:53  0:00:01  0:03:52 6209k
  1  740M    1 10.1M    0     0  3757k      0  0:03:21  0:00:02  0:03:19 5400k
  2  740M    2 18.5M    0     0  5035k      0  0:02:30  0:00:03  0:02:27 6563k
  3  740M    3 27.4M    0     0  5409k      0  0:02:20  0:00:05  0:02:15 6507k
…
75  740M   75  560M    0     0  2723k      0  0:04:38  0:03:30  0:01:08  560k
76  740M   76  564M    0     0  2730k      0  0:04:37  0:03:31  0:01:06  710kcurl: (56) OpenSSL SSL_read: SSL_ERROR_SYSCALL, errno 104

76  740M   76  565M    0     0  2464k      0  0:05:07  0:03:55  0:01:12  300k
curl: (23) Failed writing body (4096 != 16384)
Iteration 1: Trying to access nfcLease.info.entity ...
HttpNfcLeaseComplete succeeded
Uploading template sles12sp3_1.0.2018-12-10_10g into phvvvcs005.example.com-SK1/T002-RZ-WSI done successfully