Fix error in Geometry by Expression

We can't depend on preparing the expression, as we may not have
enough context to do this during the algorithm preparation step

Author: nyalldawson

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

@@ -94,11 +94,7 @@ def prepareAlgorithm(self, parameters, context, feedback):
             return False
 
         self.expression_context = self.createExpressionContext(parameters, context)
-
-        if not self.expression.prepare(self.expression_context):
-            feedback.reportErro(
-                self.tr('Evaluation error: {0}').format(self.expression.evalErrorString()))
-            return False
+        self.expression.prepare(self.expression_context)
 
         return True