Eeschema: Verify path can be made relative before asking in plotter dialog

CHANGED: When the output path is set in the plotter dialog, the user is
asked if they want to make the path relative to the project.  The old
code would ask the user if they wanted to do this, and then if it
failed would present an error dialog.  The new code tries it first on
a copy, and only if it works does the user get asked if they want
to do it.

Related to issue https://gitlab.com/kicad/code/kicad/issues/5263 except
that is for Pcbnew, and this fixes the same problem in Eeschema.  Issue
5263 is fixed with Merge Request:
https://gitlab.com/kicad/code/kicad/-/merge_requests/370

eeschema/dialogs/dialog_plot_schematic.cpp

@@ -156,17 +156,20 @@ void DIALOG_PLOT_SCHEMATIC::OnOutputDirectoryBrowseClicked( wxCommandEvent& even
     wxFileName fn( Prj().AbsolutePath( m_parent->Schematic().Root().GetFileName() ) );
     wxString defaultPath = fn.GetPathWithSep();
     wxString msg;
-    msg.Printf( _( "Do you want to use a path relative to\n\"%s\"" ), GetChars( defaultPath ) );
+    wxFileName relPathTest; // Used to test if we can make the path relative
 
-    wxMessageDialog dialog( this, msg, _( "Plot Output Directory" ),
-                            wxYES_NO | wxICON_QUESTION | wxYES_DEFAULT );
+    relPathTest.Assign( dirDialog.GetPath() );
 
-    // relative directory selected
-    if( dialog.ShowModal() == wxID_YES )
+    // Test if making the path relative is possible before asking the user if they want to do it
+    if( relPathTest.MakeRelativeTo( defaultPath ) )
     {
-        if( !dirName.MakeRelativeTo( defaultPath ) )
-            wxMessageBox( _( "Cannot make path relative (target volume different from file "
-                             "volume)!" ), _( "Plot Output Directory" ), wxOK | wxICON_ERROR );
+        msg.Printf( _( "Do you want to use a path relative to\n\"%s\"" ), GetChars( defaultPath ) );
+
+        wxMessageDialog dialog( this, msg, _( "Plot Output Directory" ),
+                                wxYES_NO | wxICON_QUESTION | wxYES_DEFAULT );
+
+        if( dialog.ShowModal() == wxID_YES )
+            dirName.MakeRelativeTo( defaultPath );
     }
 
     m_outputDirectoryName->SetValue( dirName.GetFullPath() );