Confusing errors raised when missing configuration

[Maxim-Filimonov]

Environment

• Elixir & Erlang versions (elixir --version): Erlang/OTP 23 [erts-11.2.2.2] [source] [64-bit] [smp:6:6] [ds:6:6:10] [async-threads:1] [hipe]

Elixir 1.11.4 (compiled with Erlang/OTP 23)

• ExAws version mix deps |grep ex_aws

• ex_aws (Hex package) (mix) locked at 2.2.3 (ex_aws) 8ddf1da9

• ex_aws_s3 (Hex package) (mix) locked at 2.2.0 (ex_aws_s3) 15175c61

• ex_aws_ses (Hex package) (mix) locked at 2.1.1 (ex_aws_ses) 86c8a7b1

• HTTP client version. IE for hackney do mix deps | grep hackney

• hackney 1.17.4 (Hex package) (rebar3) locked at 1.17.4 (hackney) de16ff49

Current behavior

I was running in an environment where access_key_id, secret_access_key, and region were not configured properly. When running test in this env ex_aws gives confusing error which is hard to decipher:

 ** (exit) exited in: GenServer.call(ExAws.Config.AuthCache, {:refresh_auth, %{}}, 30000)
         ** (EXIT) an exception was raised:
             ** (KeyError) key :json_codec not found in: %{}
                 (ex_aws 2.2.3) lib/ex_aws/instance_meta.ex:66: ExAws.InstanceMeta.instance_role_credentials/1
                 (ex_aws 2.2.3) lib/ex_aws/instance_meta.ex:72: ExAws.InstanceMeta.security_credentials/1
                 (ex_aws 2.2.3) lib/ex_aws/config/auth_cache.ex:116: ExAws.Config.AuthCache.refresh_auth_now/2
                 (ex_aws 2.2.3) lib/ex_aws/config/auth_cache.ex:45: ExAws.Config.AuthCache.handle_call/3
                 (stdlib 3.14.2.1) gen_server.erl:715: :gen_server.try_handle_call/4
                 (stdlib 3.14.2.1) gen_server.erl:744: :gen_server.handle_msg/6
                 (stdlib 3.14.2.1) proc_lib.erl:226: :proc_lib.init_p_do_apply/3

Expected behavior

I would expect a user friendly error which would point to missing keys in configuration.

[bernardd]

Yep, I'd definitely agree that improved errors would make some of this stuff a lot easier. I've hit almost exactly this one myself. PRs to that effect would be most welcome.