Methods returning null provide no error information

[sedano-j]

Issue description:
As already mentioned in #616 , many methods can return null, one example which has happened lately is that
createWorkItem (from WorkItemTrackingApi) returns null and provides no information why it failed to create a new work item.

The corresponding documentation page also only mentions the successful 200 response and nothing regarding a possible error.

Package version:
14.1.0 (altough it happened in previous versions as well, before upgrading)

Steps to reproduce:

• create fresh Azure DevOps project (uses the 'Basic' process)
• call workItemTrackingApi.createWorkItem(...)

The createWorkItem(...) call sets the following fields:

• '/fields/System.Description'
• '/fields/System.Title'
• '/fields/Microsoft.VSTS.Common.Priority'
• '/fields/Microsoft.VSTS.Scheduling.OriginalEstimate'

Also tried with just title and description, same outcome.