rename your capx as .zip
look for the missing texture and compare with any reference in the .caproj (open it with a text editor)
the .caproj is an xml file which reference all plugins, behavior, objecttypes, and project parameters of your project.
It is read on opening and if you read the error message it just means that you have a ressource missing.
You can either delete the reference in the xml (if you know xml well enough) or recreate a placeholder png you would put on the texture folder and try opening the caproj with construct2