cli: Fix output in case of server error + report timeout properly

[tfg13]

If the server returned any error, the client would not print it, but instead say "invalid JSON response", because it tried to unmarshal the empty response data.

Also fixing a minor problem where it did not report the correct timeout in the error message.

Test Plan: (provoking a timeout error in the server) Before:

> contest % go run ./cmds/clients/contestcli status 14
Requesting URL http://localhost:8080/status with requestor ID 'contestcli-http'
  with params:
    jobID: [14]
    requestor: [contestcli-http]

The server responded with status 400 Bad Request
cannot decode json response: unexpected end of JSON input
exit status 1

Now:

> contest % go run ./cmds/clients/contestcli status 14
Requesting URL http://localhost:8080/status with requestor ID 'contestcli-http'
  with params:
    jobID: [14]
    requestor: [contestcli-http]

The server responded with status 400 Bad Request
{
 "ServerID": "",
 "Data": {
  "Status": null
 },
 "Err": "Status failed: time out waiting for response after 3s"
}

[codecov[bot]]

Codecov Report

Merging #268 (28a920b) into master (307dfa0) will increase coverage by 0.05%. The diff coverage is 26.31%.

@@            Coverage Diff             @@
##           master     #268      +/-   ##
==========================================
+ Coverage   66.45%   66.50%   +0.05%     
==========================================
  Files         125      125              
  Lines        5959     5965       +6     
==========================================
+ Hits         3960     3967       +7     
+ Misses       1531     1525       -6     
- Partials      468      473       +5     

Flag	Coverage Δ
e2e	45.23% <26.31%> (-0.06%)	:arrow_down:
integration	60.24% <0.00%> (+0.32%)	:arrow_up:
unittests	43.48% <0.00%> (+0.08%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/api/api.go	69.92% <0.00%> (ø)
pkg/transport/http/http.go	48.95% <27.77%> (-3.27%)	:arrow_down:
pkg/runner/test_runner.go	93.19% <0.00%> (+0.70%)	:arrow_up:
plugins/storage/rdbms/init.go	58.46% <0.00%> (+1.53%)	:arrow_up:
pkg/jobmanager/jobmanager.go	78.74% <0.00%> (+1.57%)	:arrow_up:
pkg/cerrors/cerrors.go	33.33% <0.00%> (+16.66%)	:arrow_up:

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