Fix #20160 - Skip tables that cannot be read in SQL structure export - #20379

Description

When a single table cannot be read (for example an InnoDB table on a server started with skip-innodb), the whole SQL export failed with an Error reading structure for table error instead of exporting the readable tables. The export loop now skips a table whose columns cannot be read and continues, the same way #20224 handles the data export.

Fixes #20160.

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