[processing] Log use of ballpark transforms to processing log instead

of to the main window message bar

More processing algorithms will require this, but it's a template for
the others to be adapted

src/analysis/processing/qgsalgorithmtransform.cpp

@@ -102,6 +102,8 @@ QgsFeatureList QgsTransformAlgorithm::processFeature( const QgsFeature &f, QgsPr
     if ( !mCoordOp.isEmpty() )
       mTransformContext.addCoordinateOperation( sourceCrs(), mDestCrs, mCoordOp );
     mTransform = QgsCoordinateTransform( sourceCrs(), mDestCrs, mTransformContext );
+
+    mTransform.disableFallbackOperationHandler( true );
   }
 
   if ( feature.hasGeometry() )
@@ -117,6 +119,14 @@ QgsFeatureList QgsTransformAlgorithm::processFeature( const QgsFeature &f, QgsPr
       {
         feature.clearGeometry();
       }
+
+      if ( !mWarnedAboutFallbackTransform && mTransform.fallbackOperationOccurred() )
+      {
+        feedback->reportError( QObject::tr( "An alternative, ballpark-only transform was used when transforming coordinates for one or more features. "
+                                            "(Possibly an incorrect choice of operation was made for transformations between these reference systems - check "
+                                            "that the selected operation is valid for the full extent of the input layer.)" ) );
+        mWarnedAboutFallbackTransform = true; // only warn once to avoid flooding the log
+      }
     }
     catch ( QgsCsException & )
     {

src/analysis/processing/qgsalgorithmtransform.h

@@ -59,6 +59,7 @@ class QgsTransformAlgorithm : public QgsProcessingFeatureBasedAlgorithm
     QgsCoordinateTransform mTransform;
     QgsCoordinateTransformContext mTransformContext;
     QString mCoordOp;
+    bool mWarnedAboutFallbackTransform = false;
 
 };