[patch] Raise InternalServerError while retrieving table DDL

Hi Hackers,

While doing the DDL patch we found out that the code was not working
properly when errors happened during SQL execution:

One example of this can be found in the file:

web/pgadmin/browser/server_groups/servers/databases/schemas/tables/__init__.py

The function '*_formatter*' that returns internal_server_error when an
error occur executing SQL
Nevertheless the '*sql*' function uses the output of '*_formatter*' during
the execution without checking it.

To solve this issue we raise an InternalServerError exception that we catch
in the '*sql*' function instead of returning an error message.

Thanks
Joao & Sarah