New project from exported one - empty error message when no file is selected

[FabianCretton]

I wanted to create a new project from an exported one, and I pressed on "Create project" before choosing the file.
There was an error message, but empty:

[tpluscode]

RE zazuko/rdf-validate-shacl#67

The shape used is an alternative (sh:xone) of the 3 variants for creating project: mapping, external, import (excerpt)

</shape/cube-project/create>
  sh:xone (
    [
       sh:property [
         sh:path cc:projectSourceKind ;
         dash:hidden true ;
         sh:hasValue <https://cube-creator.zazuko.com/vocab#projectSourceKind/ExportedProject> ;
       ], [
         sh:name "Project data" ;
         sh:path cc:export ;
         sh:minCount 1 ;
         sh:maxCount 1 ;
         dash:editor cc:dashFileUpload ;
      ]
    ]
  ) ;
.

The error is not correctly communicated from the nested shape and it's an issue in multiple places

[l00mi]

Will be solved with #947, closing this one for now.