Show actual original coordinate in QgsCsException message

We were incorrectly showing the "transformed" coordinate result
in the exception message, which usually ended up inf, inf after
the transform fails. This lead to a useless 'forward transform of
(inf, inf)' message in the exception, which was misleading
as it looks like we were trying to transform a (inf, inf) point.

Now we get the (useful!) actual coordinate we were trying
to transform in the exception message.

src/core/proj/qgscoordinatetransform.cpp

@@ -892,7 +892,7 @@ void QgsCoordinateTransform::transformCoords( int numPoints, double *x, double *
 
     for ( int i = 0; i < numPoints; ++i )
     {
-      points += QStringLiteral( "(%1, %2)\n" ).arg( x[i], 0, 'f' ).arg( y[i], 0, 'f' );
+      points += QStringLiteral( "(%1, %2)\n" ).arg( xprev[i], 0, 'f' ).arg( yprev[i], 0, 'f' );
     }
 
     const QString dir = ( direction == Qgis::TransformDirection::Forward ) ? QObject::tr( "forward transform" ) : QObject::tr( "inverse transform" );