Offer a damaged project's previous version, or what can be read of it

A project file that could not be read showed "The file ... does not
appear to be a QElectroTech project file", for a .qetz missing one
picture as for a picture file renamed .qet: wrong, and no way out.

Now the message says what is damaged (QETProject::openError()) and
offers, when there are any:
- the newest previous version kept beside it (PreviousVersions);
- for a .qetz, what can be read of it (QetContainer::recover()), then a
  list of what was left out.
Either opens as a new project, "<name> (recovered)" beside the file,
numbered if that is taken, not saved yet. The damaged file is never
moved or written.

Checked in the program (scripts/qet-gui-dialog.sh, File > Open on a
.qetz whose picture is missing, a previous version beside it): the
dialog says which picture; "Recover what can be read" opens plant
(recovered).qetz without the picture, saved with Ctrl+S it opens again;
"Open the version saved ..." opens plant (recovered 2).qetz with its
picture; plant.qetz unchanged (same sha256). ctest 107/107.

Stacked on "Keep the previous versions of a project when saving it" and
"Add recovering what can be read of a damaged .qetz".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sources/qetproject.cpp

@@ -515,6 +515,7 @@ QETProject::ProjectState QETProject::openFile(QFile *file)
 	if (!read)
 	{
 		qWarning().noquote() << "Unable to read" << fi.fileName() << ":" << read_error;
+		m_open_error = read_error;
 		m_container_pictures.clear();
 		m_folio_parts.clear();
 		if(opened_here) {
@@ -603,6 +604,16 @@ QETProject::ProjectState QETProject::state() const
 	return(m_state);
 }
 
+/**
+	@brief QETProject::openError
+	@return why the file could not be read, when state() is
+	XmlParsingFailed: "picture ... is missing", say; empty otherwise
+*/
+QString QETProject::openError() const
+{
+	return m_open_error;
+}
+
 /**
 	@return la liste des schemas de ce projet
 */