[Feature Request]: Make error message more helpful when `TaskItem.ItemSpec` is not set

[MangelMaxime]

Summary

Hello,

Improve error message if ItemSpec is missing on a TaskItem

Background and Motivation

When creating a custom MSBuild task, it took me several hours to figure out why I was getting this error:

error MSB4028: The "EasyBuild.PackageReleaseNotes.Tasks.GetCurrentReleaseTask" task's outputs could not be retrieved from the "CurrentRelease" parameter. Parameter "includeEscaped" cannot have zero length.

The reason was that I didn't set the TaskItem.ItemSpec value.

Proposed Feature

Improve the error message to mention that ItemSpec should be set.

The "EasyBuild.PackageReleaseNotes.Tasks.GetCurrentReleaseTask" task's outputs could not be retrieved from the "CurrentRelease" parameter. Please set the value of ItemSpec, in your task.

Alternative Designs

Another solution would be to change the constructor of TaskItem to take a mandatory ItemSpec value, but I suspect it would be too big of a breaking change.