Fix our spec.s as per elvis_core and output more context on error

The test is a dumb one, not unlike the prior version of it,
but it makes sure we don't crash on function call (mimics
rebar3) and on io:format'ing it

src/rebar3_lint_prv.erl

@@ -22,23 +22,25 @@ init(State) ->
                           {desc, "A rebar linter plugin based on elvis"}]),
     {ok, rebar_state:add_provider(State, Provider)}.
 
--spec do(rebar_state:t()) -> {ok, rebar_state:t()} | {error, string()}.
+-spec do(rebar_state:t()) -> {ok, rebar_state:t()} | {error, {string(), any()}}.
 do(State) ->
     Elvis = get_elvis_config(State),
     _ = rebar_log:log(info, "elvis analysis starting, this may take a while...", []),
     try elvis_core:rock(Elvis) of
         ok ->
             {ok, State};
-        {fail, _} ->
-            {error, "Linting failed"}
+        {fail, [{throw, Error} | _]} ->
+            rebar_api:abort("elvis_core threw an exception: ~p", [Error]);
+        {fail, Reason} ->
+            {error, {"Linting failed with ~p", Reason}}
     catch
         Error ->
             rebar_api:abort("elvis_core threw an exception: ~p", [Error])
     end.
 
--spec format_error(any()) -> iolist().
-format_error(Reason) ->
-    io_lib:format("~p", [Reason]).
+-spec format_error({string(), any()}) -> iolist().
+format_error({Message, Reason}) ->
+    io_lib:format(Message, [Reason]).
 
 -spec get_elvis_config(rebar_state:t()) -> elvis_config:configs().
 get_elvis_config(State) ->

test/test_app_SUITE.erl

@@ -19,4 +19,6 @@ test_app(_Config) ->
                         [#{dirs => ["src"],
                            filter => "*.erl",
                            rules => [{elvis_style, max_function_length, #{max_length => 1}}]}]),
-    {error, "Linting failed"} = rebar3_lint_prv:do(BadState).
+    {error, {Message = "Linting failed with ~p", Reason}} = rebar3_lint_prv:do(BadState),
+    % Check that this doesn't throw
+    io:format("~p", [rebar3_lint_prv:format_error({Message, Reason})]).