Allow (visually) exporting the list of known API error codes

The goal, as stated in the change, is to easy copy-pasting
to developer.kivra.com, thus reducing the chance of
inconsistency

README.md

@@ -127,3 +127,25 @@ The following message contains the same information, but is more concise:
 Run the linter to validate the error definitions in [`api-errors.json`](./api-errors.json):
 
     make lint
+
+## Export for use in `developer.kivra.com`
+
+A helper function is made available to allow copy-pasting the entire list for
+acceptance in <https://developer.kivra.com>.
+
+### Erlang
+
+Boot up an Erlang shell
+
+```console
+rebar3 shell
+```
+
+Call the `kivra_api_errors:to_developer_kivra_com/0` function:
+
+```erlang
+kivra_api_errors:to_developer_kivra_com().
+```
+
+Whatever is between `<copy-paste>` and `</copy-paste>` is deemed usable
+(YML-formatted) in <https://developer.kivra.com>.

erlang/kivra_api_errors.erl

@@ -5,7 +5,8 @@
 -export([
   load/0,
   load/1,
-  from_code/1, from_code/2, from_code/3
+  from_code/1, from_code/2, from_code/3,
+  to_developer_kivra_com/0
 ]).
 
 %%%_* Types ===================================================================
@@ -37,6 +38,26 @@ load(Config) ->
       Error
   end.
 
+-spec to_developer_kivra_com() -> ok.
+to_developer_kivra_com() ->
+  ErrorsFile = filename:join([code:priv_dir(?MODULE), <<"api-errors.json">>]),
+  {ok, ErrorsAsJSON} = file:read_file(ErrorsFile),
+  Errors = jiffy:decode(ErrorsAsJSON, [return_maps]),
+  ErrorsList = maps:to_list(Errors),
+  SortedErrorsList = lists:sort(fun ({CodeLeft, _}, {CodeRight, _}) -> CodeLeft < CodeRight end, ErrorsList),
+
+  io:format("Copy-paste to developer.kivra.com's swagger.yml~n"),
+  io:format("<copy-paste>~n"),
+  io:format("    | Code | Short Message | Long Message |~n"),
+  io:format("    | ---- | ------------- | ------------ |~n"),
+  lists:foreach(
+    fun ({Code, #{<<"short_message">> := ShortMessage, <<"long_message">> := LongMessage}}) ->
+      io:format("    | ~ts | ~ts | ~ts |~n", [Code, ShortMessage, LongMessage])
+    end,
+    SortedErrorsList
+  ),
+  io:format("</copy-paste>~n").
+
 -spec from_code(binary() | pos_integer()) -> {ok, {status_code(), payload_map() | payload_kv()}} | {error, notfound}.
 from_code(ErrorCode) when is_integer(ErrorCode) ->
   from_code(integer_to_binary(ErrorCode));