Refactor fields configuration in the modeler does not insist on layer

Inside the modeler, there is not enough knowledge about the layer on
which the refactor fields algorithm will run.
Let's be graceful with error messages here therefore.

python/plugins/processing/algs/qgis/FieldsMapper.py

@@ -137,8 +137,8 @@ class FieldsMapper(QgisFeatureBasedAlgorithm):
             if expression.hasParserError():
                 raise QgsProcessingException(
                     self.tr(u'Parser error in expression "{}": {}')
-                    .format(str(expression.expression()),
-                            str(expression.parserErrorString())))
+                    .format(expression.expression(),
+                            expression.parserErrorString()))
             self.expressions.append(expression)
         return True
 
@@ -160,8 +160,8 @@ class FieldsMapper(QgisFeatureBasedAlgorithm):
             if expression.hasEvalError():
                 raise QgsProcessingException(
                     self.tr(u'Evaluation error in expression "{}": {}')
-                        .format(str(expression.expression()),
-                                str(expression.parserErrorString())))
+                        .format(expression.expression(),
+                                expression.parserErrorString()))
             attributes.append(value)
         feature.setAttributes(attributes)
         self._row_number += 1

python/plugins/processing/algs/qgis/ui/FieldsMappingPanel.py

@@ -225,8 +225,7 @@ class FieldsMappingModel(QAbstractTableModel):
 
         self._mapping = []
         if layer is not None:
-            dp = layer.dataProvider()
-            for field in dp.fields():
+            for field in layer.fields():
                 self._mapping.append(self.newField(field))
 
         self.endResetModel()
@@ -263,6 +262,7 @@ class ExpressionDelegate(QStyledItemDelegate):
         editor.registerExpressionContextGenerator(index.model().contextGenerator())
         editor.fieldChanged.connect(self.on_expression_fieldChange)
         editor.setAutoFillBackground(True)
+        editor.setAllowEvalErrors(self.parent().dialogType == DIALOG_MODELER)
         return editor
 
     def setEditorData(self, editor, index):
@@ -303,6 +303,7 @@ class FieldsMappingPanel(BASE, WIDGET):
 
         self.layerCombo.setAllowEmptyLayer(True)
         self.layerCombo.setFilters(QgsMapLayerProxyModel.VectorLayer)
+        self.dialogType = None
 
     def configure(self):
         self.model = FieldsMappingModel()
@@ -472,7 +473,9 @@ class FieldsMappingWidgetWrapper(WidgetWrapper):
         self._layer = None
 
     def createWidget(self):
-        return FieldsMappingPanel()
+        panel = FieldsMappingPanel()
+        panel.dialogType = self.dialogType
+        return panel
 
     def postInitialize(self, wrappers):
         for wrapper in wrappers: