Use more explicit message for unknown column header when autodetecting CSV

Closes #12631

sao/src/window.js

@@ -1599,9 +1599,8 @@
             }
             else {
                 Sao.common.warning.run(
-                    Sao.i18n.gettext(
-                        'Error processing the file at field %1.', word),
-                        Sao.i18n.gettext('Error'));
+                    Sao.i18n.gettext('Unknown column header "%1"', word),
+                    Sao.i18n.gettext('Error'));
                 return false;
             }
             var node = jQuery('<li/>', {

tryton/tryton/gui/window/win_import.py

@@ -147,8 +147,9 @@ class WinImport(WinCSV):
                     name = self.fields[word][0]
                     field = word
                 else:
-                    common.warning(_('Error processing the file at field %s.')
-                        % word, _('Error'))
+                    common.warning(
+                        _('Unknown column header "%s"') % word,
+                        _('Error'))
                     return True
                 num = self.model2.append()
                 self.model2.set(num, 0, name, 1, field)