Issue warnings on empty folders

Helps keep a trimmed down elvis.config

Example output:
Warning: Searching for files in ["test/examples/2"] yielded none. Update your configuration.

src/elvis_config.erl

@@ -158,6 +158,14 @@ resolve_files(RuleGroup, Files) ->
     Dirs = dirs(RuleGroup),
     Ignore = ignore(RuleGroup),
     FilteredFiles = elvis_file:filter_files(Files, Dirs, Filter, Ignore),
+    _ = case FilteredFiles of
+            [] ->
+                Error = elvis_result:new(warn, "Searching for files in ~p yielded none. "
+                                               "Update your configuration.", [Dirs]),
+                ok = elvis_result:print_results([Error]);
+            _ ->
+                ok
+        end,
     RuleGroup#{files => FilteredFiles}.
 
 %% @doc Takes a configuration and finds all files according to its 'dirs'

src/elvis_result.erl

@@ -53,6 +53,11 @@
            error_msg => string(),
            info => list()
          }.
+-type elvis_warn() ::
+        #{
+           warn_msg => string(),
+           info => list()
+         }.
 
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 %% Public
@@ -63,15 +68,18 @@
 -spec new(item, string(), [term()]) -> item()
        ; (rule, atom(), [item()]) -> rule()
        ; (file, elvis_file:file(), [elvis_error() | rule()]) -> file()
-       ; (error, string(), string()) -> elvis_error().
+       ; (error, string(), string()) -> elvis_error()
+       ; (warn, string(), string()) -> elvis_warn().
 new(item, Msg, Info) ->
     new(item, Msg, Info, 0);
 new(rule, Name, Results) ->
     #{name => Name, items => Results};
 new(file, #{path := Path}, Rules) ->
     #{file => Path, rules => Rules};
 new(error, Msg, Info) ->
-    #{error_msg => Msg, info => Info}.
+    #{error_msg => Msg, info => Info};
+new(warn, Msg, Info) ->
+    #{warn_msg => Msg, info => Info}.
 
 -spec new(item, string(), [term()], integer()) -> item().
 new(item, Msg, Info, LineNum) ->
@@ -165,7 +173,9 @@ print_item(_Format, _File, _Name, []) ->
     ok.
 
 print_error(#{error_msg := Msg, info := Info}) ->
-    elvis_utils:error_prn(Msg, Info).
+    elvis_utils:error_prn(Msg, Info);
+print_error(#{warn_msg := Msg, info := Info}) ->
+    elvis_utils:warn_prn(Msg, Info).
 
 -spec status([file() | rule()]) -> ok | fail.
 status([]) ->

src/elvis_utils.erl

@@ -22,6 +22,7 @@
          error/2,
          error_prn/1,
          error_prn/2,
+         warn_prn/2,
          parse_colors/1
         ]).
 
@@ -162,6 +163,11 @@ error_prn(Message, Args) ->
     ColoredMessage = "{{red}}Error: {{reset}}" ++ Message ++ "{{reset}}~n",
     print(ColoredMessage, Args).
 
+-spec warn_prn(string(), [term()]) -> ok.
+warn_prn(Message, Args) ->
+    ColoredMessage = "{{magenta}}Warning: {{reset}}" ++ Message ++ "{{reset}}~n",
+    print(ColoredMessage, Args).
+
 -spec print_info(string(), [term()]) -> ok.
 print_info(Message, Args) ->
     case application:get_env(elvis_core, verbose) of
@@ -188,6 +194,7 @@ parse_colors(Message) ->
                "green-bold" => "\e[1;32m",
                "white" => "\e[0;37m",
                "white-bold" => "\e[1;37m",
+               "magenta" => "\e[1;35m",
                "reset" => "\e[0m"},
     Opts = [global, {return, list}],
     case application:get_env(elvis_core, output_format, colors) of