Backtrace when database is out of diskspace

[pjonsson]

Found this in the logs for an OWS image based on commit eda5f2b. I think this is a different issue from #1388, so putting the backtrace here for reference:

Traceback (most recent call last):
  File "/app/lib/python3.12/site-packages/sqlalchemy/engine/base.py", line 1967, in _exec_single_context
    self.dialect.do_execute(
  File "/app/lib/python3.12/site-packages/sqlalchemy/engine/default.py", line 951, in do_execute
    cursor.execute(statement, parameters)
  File "/app/lib/python3.12/site-packages/psycopg/cursor.py", line 117, in execute
    raise ex.with_traceback(None)
psycopg.errors.DiskFull: could not resize shared memory segment "/PostgreSQL.2417566142" to 4206592 bytes: No space left on device
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
  File "/app/lib/python3.12/site-packages/datacube_ows/ogc.py", line 183, in ogc_svc_impl
    return version_support.router(nocase_args)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/lib/python3.12/site-packages/datacube_ows/utils.py", line 38, in log_wrapper
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/app/lib/python3.12/site-packages/datacube_ows/wms.py", line 29, in handle_wms
    return get_map(nocase_args)
           ^^^^^^^^^^^^^^^^^^^^
  File "/app/lib/python3.12/site-packages/datacube_ows/utils.py", line 38, in log_wrapper
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/app/lib/python3.12/site-packages/datacube_ows/data.py", line 107, in get_map
    n_datasets = stacker.n_datasets()
                 ^^^^^^^^^^^^^^^^^^^^
  File "/app/lib/python3.12/site-packages/datacube_ows/loading.py", line 200, in n_datasets
    return self._layer.ows_index().count(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/lib/python3.12/site-packages/datacube_ows/index/postgis/api.py", line 201, in count
    return layer.dc.index.datasets.count(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/lib/python3.12/site-packages/datacube/index/postgis/_datasets.py", line 839, in count
    for _, count in self._do_count_by_product(query, archived=archived):
  File "/app/lib/python3.12/site-packages/datacube/index/postgis/_datasets.py", line 965, in _do_count_by_product
    count = connection.count_datasets(
            ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/lib/python3.12/site-packages/datacube/drivers/postgis/_api.py", line 831, in count_datasets
    num = self._connection.scalar(select_query)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/lib/python3.12/site-packages/sqlalchemy/engine/base.py", line 1308, in scalar
    return meth(
           ^^^^^
  File "/app/lib/python3.12/site-packages/sqlalchemy/sql/elements.py", line 544, in _execute_on_scalar
    return self._execute_on_connection(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/lib/python3.12/site-packages/sqlalchemy/sql/elements.py", line 526, in _execute_on_connection
    return connection._execute_clauseelement(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/lib/python3.12/site-packages/sqlalchemy/engine/base.py", line 1641, in _execute_clauseelement
    ret = self._execute_context(
          ^^^^^^^^^^^^^^^^^^^^^^
  File "/app/lib/python3.12/site-packages/sqlalchemy/engine/base.py", line 1846, in _execute_context
    return self._exec_single_context(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/lib/python3.12/site-packages/sqlalchemy/engine/base.py", line 1986, in _exec_single_context
    self._handle_dbapi_exception(
  File "/app/lib/python3.12/site-packages/sqlalchemy/engine/base.py", line 2355, in _handle_dbapi_exception
    raise sqlalchemy_exception.with_traceback(exc_info[2]) from e
  File "/app/lib/python3.12/site-packages/sqlalchemy/engine/base.py", line 1967, in _exec_single_context
    self.dialect.do_execute(
  File "/app/lib/python3.12/site-packages/sqlalchemy/engine/default.py", line 951, in do_execute
    cursor.execute(statement, parameters)
  File "/app/lib/python3.12/site-packages/psycopg/cursor.py", line 117, in execute
    raise ex.with_traceback(None)
sqlalchemy.exc.OperationalError: (psycopg.errors.DiskFull) could not resize shared memory segment "/PostgreSQL.2417566142" to 4206592 bytes: No space left on device
[SQL: SELECT count(odc.dataset.id) AS count_1

OWS can't do much when the database isn't functioning, this issue is just about catching the exception in the right place so we don't get a long backtrace in the logs.