Gracefully handle NotSupportedError during schema introspection

[gavinhodge]

In certain circumstances, a django.db.utils.NotSupportedError can be thrown during schema introspection. If this happens, the exception is unhandled and Django returns a 500 response instead of displaying query results.

To reproduce requires:

• Postgresql running in recovery mode (typically a hot standby server). This can be simulated without a hot standby server
• An unlogged table on the primary (ALTER TABLE table_name SET UNLOGGED)
• Attempt to execute a query (e.g. SELECT 1) against the standby server and receive a 500 error page

Exception detail:

NotSupportedError
cannot access temporary or unlogged relations during recovery

The issue can be worked around by specifying the affected tables in the EXPLORER_SCHEMA_EXCLUDE_TABLE_PREFIXES setting, but this change makes a workaround unnecessary.