Make backend sync errors survive to the session.

The backends were using qof_backend_get_error() to test for sync errors.
This function clears the error, so the tests resulted in the error being
cleared before the session could see it and so it thought that the sync
had succeeded.

Replace those uses of qof_backend_get_error() with a new function
qof_backend_check_error() that doesn't clear the error.

src/backend/dbi/gnc-backend-dbi.c

@@ -1685,7 +1685,7 @@ gnc_dbi_safe_sync_all( QofBackend *qbe, QofBook *book )
     be->primary_book = book;
 
     gnc_sql_sync_all( &be->sql_be, book );
-    if ( ERR_BACKEND_NO_ERR != qof_backend_get_error( qbe ) )
+    if (qof_backend_check_error (qbe))
     {
         conn_table_operation( (GncSqlConnection*)conn, table_list,
                               rollback );

src/backend/sql/gnc-backend-sql.c

@@ -512,6 +512,7 @@ gnc_sql_sync_all( GncSqlBackend* be, /*@ dependent @*/ QofBook *book )
     }
     else
     {
+        if (!qof_backend_check_error ((QofBackend*)be))
             qof_backend_set_error( (QofBackend*)be, ERR_BACKEND_SERVER_ERR );
         is_ok = gnc_sql_connection_rollback_transaction( be->conn );
     }

src/backend/xml/io-gncxml-v2.c

@@ -1670,8 +1670,7 @@ gnc_book_write_accounts_to_xml_file_v2(
     if (out && fclose(out))
         success = FALSE;
 
-    if (!success
-            && qof_backend_get_error(be) == ERR_BACKEND_NO_ERR)
+    if (!success && !qof_backend_check_error(be))
     {
 
         /* Use a generic write error code */
@@ -2186,4 +2185,3 @@ gnc_xml2_parse_with_subst (FileBackend *fbe, QofBook *book, GHashTable *subst)
 
     return success;
 }
-

src/libqof/qof/qofbackend.c

@@ -64,6 +64,13 @@ qof_backend_get_error (QofBackend *be)
     return err;
 }
 
+gboolean
+qof_backend_check_error (QofBackend *be)
+{
+    g_return_val_if_fail (be != NULL, TRUE);
+    return be->last_err != ERR_BACKEND_NO_ERR;
+}
+
 void
 qof_backend_set_message (QofBackend *be, const char *format, ...)
 {

src/libqof/qof/qofbackend.h

@@ -166,6 +166,14 @@ void qof_backend_set_error (QofBackend *be, QofBackendError err);
  */
 QofBackendError qof_backend_get_error (QofBackend *be);
 
+/** Report if the backend is in an error state.
+ *  Since get_error resets the error state, its use for branching as the backend
+ *  bubbles back up to the session would make the session think that there was
+ *  no error.
+ * \param be The backend being tested.
+ * \return TRUE if the backend has an error set.
+ */
+gboolean qof_backend_check_error (QofBackend *be);
 
 /** \brief Load a QOF-compatible backend shared library.