Gracefully handle exhausted postgresql connection slots

The default configuration of postgresql has max_connections = 100. The default configuration of Debusine seems to exceed that value under load. Typical traceback:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/django/db/backends/base/base.py", line 289, in ensure_connection
    self.connect()
    ^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/django/utils/asyncio.py", line 26, in inner
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/django/db/backends/base/base.py", line 270, in connect
    self.connection = self.get_new_connection(conn_params)
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/django/utils/asyncio.py", line 26, in inner
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/django/db/backends/postgresql/base.py", line 275, in get_new_connection
    connection = self.Database.connect(**conn_params)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/psycopg2/__init__.py", line 122, in connect
    conn = _connect(dsn, connection_factory=connection_factory, **kwasync)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The above exception (connection to server on socket "/var/run/postgresql/.s.PGSQL.5432" failed: FATAL:  remaining connection slots are reserved for non-replication superuser
+connections
) was the direct cause of the following exception:
  File "/usr/lib/python3/dist-packages/django/core/handlers/exception.py", line 55, in inner
    response = get_response(request)
               ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/debusine/server/middlewares/scopes.py", line 66, in __call__
    context.set_scope(self.get_scope(scope_name))
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/debusine/server/middlewares/scopes.py", line 77, in get_scope
    return get_object_or_404(Scope, name=name)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/django/shortcuts.py", line 85, in get_object_or_404
    return queryset.get(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/django/db/models/query.py", line 633, in get
    num = len(clone)
          ^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/django/db/models/query.py", line 380, in __len__
    self._fetch_all()
    ^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/django/db/models/query.py", line 1881, in _fetch_all
    self._result_cache = list(self._iterable_class(self))
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/django/db/models/query.py", line 91, in __iter__
    results = compiler.execute_sql(

  File "/usr/lib/python3/dist-packages/django/db/models/sql/compiler.py", line 1560, in execute_sql
    cursor = self.connection.cursor()
             ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/django/utils/asyncio.py", line 26, in inner
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/django/db/backends/base/base.py", line 330, in cursor
    return self._cursor()
           ^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/django/db/backends/base/base.py", line 306, in _cursor
    self.ensure_connection()
    ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/django/utils/asyncio.py", line 26, in inner
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/django/db/backends/base/base.py", line 288, in ensure_connection
    with self.wrap_database_errors:
    ^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/django/db/utils.py", line 91, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/django/db/backends/base/base.py", line 289, in ensure_connection
    self.connect()
    ^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/django/utils/asyncio.py", line 26, in inner
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/django/db/backends/base/base.py", line 270, in connect
    self.connection = self.get_new_connection(conn_params)
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/django/utils/asyncio.py", line 26, in inner
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/django/db/backends/postgresql/base.py", line 275, in get_new_connection
    connection = self.Database.connect(**conn_params)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/psycopg2/__init__.py", line 122, in connect
    conn = _connect(dsn, connection_factory=connection_factory, **kwasync)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Exception Type: OperationalError at ...
Exception Value: connection to server on socket "/var/run/postgresql/..." failed: FATAL:  remaining connection slots are reserved for non-replication superuser connections

To make matters worse, every such invocation generates a mail.