[FEATURE] Error message if prop not present in layout

[mstranne]

DESCRIPTION

It would be very nice to get an error message if the property in retval = window ID -get value prop=PROPNAME; is not present in the layout. Currently you just get an empty value returned.

(Do not write below this line)

---

DEVS' SECTION

ANALYSIS

Should be straightforward.

IMPLEMENTATION STEPS

(see also our Wiki for implementation guidelines)

• Implement the necessary changes in a new branch created here on GitHub
• Test your implementation

DOCUMENTATION STEPS

(see also our Wiki for further information)

• Update the changes log
• Add comments to your implementation
• Add Doxygen documentation comments
• Create or update the documentation articles (*.NHLP and *.NDB files, if needed)
• Update the language strings (*.NLNG files, if needed)

PULL REQUEST

• Create a pull request for your changes
• Fill out the template
• Assign @numere-org/maintainers as reviewers