[processing] Don't throw an exception if an expression cannot

be prepared

There's cases where this happens without reflecting an invalid
expression. So we can try to prepare, but not abort if the
preparation fails.

Fixes #18103

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

@@ -123,9 +123,7 @@ def processAlgorithm(self, parameters, context, feedback):
         if layer is not None:
             exp_context.appendScope(QgsExpressionContextUtils.layerScope(layer))
 
-        if not expression.prepare(exp_context):
-            raise QgsProcessingException(
-                self.tr('Evaluation error: {0}').format(expression.parserErrorString()))
+        expression.prepare(exp_context)
 
         features = source.getFeatures()
         total = 100.0 / source.featureCount() if source.featureCount() else 0

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

@@ -116,9 +116,7 @@ def processAlgorithm(self, parameters, context, feedback):
             raise QgsProcessingException(expression.parserErrorString())
 
         expressionContext = self.createExpressionContext(parameters, context, source)
-        if not expression.prepare(expressionContext):
-            raise QgsProcessingException(
-                self.tr('Evaluation error: {0}').format(expression.evalErrorString()))
+        expression.prepare(expressionContext)
 
         fields = QgsFields()
         fields.append(QgsField('id', QVariant.Int, '', 10, 0))