Application fails to start when external auth provider is unavailable

When starting the application, and a given Auth Provider setting (specifically the discovery url) is either incorrect or the host is not available then the application will fail to start:

** (Mix) Could not start application lightning: Lightning.Application.start(:normal, [:timex]) returned an error: shutdown: failed to start child: Cachex
    ** (EXIT) an exception was raised:
        ** (MatchError) no match of right hand side value: {:error, {:shutdown, {:failed_to_start_child, Cachex.Services.Incubator, {:shutdown, {:failed_to_start_child, Lightning.AuthProviders.CacheWarmer, {:function_clause, [{Map, :from_struct, [error: %HTTPoison.Error{id: nil, reason: :econnrefused}], [file: 'lib/map.ex', line: 946]}, {Lightning.AuthProviders.Handler, :validate_opts, 1, [file: 'lib/lightning/auth_providers/handler.ex', line: 115]}, {Lightning.AuthProviders.Handler, :new, 2, [file: 'lib/lightning/auth_providers/handler.ex', line: 38]}, {Lightning.AuthProviders.CacheWarmer, :execute, 1, [file: 'lib/lightning/auth_providers/cache_warmer.ex', line: 25]}, {Lightning.AuthProviders.CacheWarmer, :handle_info, 2, [file: 'lib/cachex/warmer.ex', line: 84]}, {Lightning.AuthProviders.CacheWarmer, :init, 1, [file: 'lib/cachex/warmer.ex', line: 72]}, {:gen_server, :init_it, 2, [file: 'gen_server.erl', line: 423]}, {:gen_server, :init_it, 6, [file: 'gen_server.erl', line: 390]}, {:proc_lib, :init_p_do_apply, 3, [file: 'proc_lib.erl', line: 226]}]}}}}}}
            (cachex 3.4.0) lib/cachex.ex:301: Cachex.start_link/1
            (stdlib 3.17) supervisor.erl:414: :supervisor.do_start_child_i/3
            (stdlib 3.17) supervisor.erl:400: :supervisor.do_start_child/2
            (stdlib 3.17) supervisor.erl:384: anonymous fn/3 in :supervisor.start_children/2
            (stdlib 3.17) supervisor.erl:1250: :supervisor.children_map/4
            (stdlib 3.17) supervisor.erl:350: :supervisor.init_children/2
            (stdlib 3.17) gen_server.erl:423: :gen_server.init_it/2
            (stdlib 3.17) gen_server.erl:390: :gen_server.init_it/6
            (stdlib 3.17) proc_lib.erl:226: :proc_lib.init_p_do_apply/3

Expected behavior

In the case of an unavailable provider, we need to fail gracefully with a warning.

Additional context

We have discussed previously, to only use the discovery url as a way to seed the endpoint urls - but taking this approach we won't be exposed to this kind of error scenario.

Start by reading lib/lightning/auth_providers/handler.ex and lib/lightning/auth_providers/cache_warmer.ex, then reproduce startup with an unavailable or invalid discovery URL. Trace how the HTTPoison error reaches validation and confirm that startup completes with a warning instead of failing the Cachex child.