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

### 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.