Add debug message in case of failure in QgsSLConnect::sqlite3_close()

and in QgsSLConnect::sqlite3_close_v2() too. This can make spotting
issues like the one fixed in b0f2b97
easier.

(cherry-picked from b253a29)

Author: rouault

src/core/qgsslconnect.cpp

@@ -14,6 +14,7 @@
  ***************************************************************************/
 
 #include "qgsslconnect.h"
+#include "qgslogger.h"
 
 #include <sqlite3.h>
 #include <spatialite.h>
@@ -52,6 +53,11 @@ int QgsSLConnect::sqlite3_close( sqlite3 *db )
     spatialite_cleanup_ex( mSLconns.take( db ) );
 #endif
 
+  if ( res != SQLITE_OK )
+  {
+    QgsDebugMsg( QString( "sqlite3_close() failed: %1" ).arg( res ) );
+  }
+
   return res;
 }
 
@@ -85,5 +91,10 @@ int QgsSLConnect::sqlite3_close_v2( sqlite3 *db )
     spatialite_cleanup_ex( mSLconns.take( db ) );
 #endif
 
+  if ( res != SQLITE_OK )
+  {
+    QgsDebugMsg( QString( "sqlite3_close() failed: %1" ).arg( res ) );
+  }
+
   return res;
 }