[processing] More verbose error when unsupported SAGA version found

(cherry-picked from 16106b8)

Author: nyalldawson

python/plugins/processing/algs/saga/SagaAlgorithmProvider.py

@@ -43,6 +43,8 @@
 pluginPath = os.path.normpath(os.path.join(
     os.path.split(os.path.dirname(__file__))[0], os.pardir))
 
+REQUIRED_VERSION = '2.3.'
+
 
 class SagaAlgorithmProvider(QgsProcessingProvider):
 
@@ -85,8 +87,8 @@ def loadAlgorithms(self):
                                      self.tr('Processing'), Qgis.Critical)
             return
 
-        if not version.startswith('2.3.'):
-            QgsMessageLog.logMessage(self.tr('Problem with SAGA installation: unsupported SAGA version found.'),
+        if not version.startswith(REQUIRED_VERSION):
+            QgsMessageLog.logMessage(self.tr('Problem with SAGA installation: unsupported SAGA version (found: {}, required: {}).').format(version, REQUIRED_VERSION),
                                      self.tr('Processing'),
                                      Qgis.Critical)
             return