Add format_error/1 to print errors nicely

src/rebar_compiler_erl.erl

@@ -6,9 +6,11 @@
          needed_files/4,
          dependencies/3,
          compile/4,
-         clean/2]).
+         clean/2,
+         format_error/1]).
 
 -include("rebar.hrl").
+-include_lib("providers/include/providers.hrl").
 
 context(AppInfo) ->
     EbinDir = rebar_app_info:ebin_dir(AppInfo),
@@ -81,7 +83,7 @@ dependencies(Source, SourceDir, Dirs) ->
             ok = file:close(Fd),
             AbsIncls;
         {error, Reason} ->
-            throw({cannot_read_file, Source, file:format_error(Reason)})
+            throw(?PRV_ERROR({cannot_read_file, Source, file:format_error(Reason)}))
     end.
 
 compile(Source, [{_, OutDir}], Config, ErlOpts) ->
@@ -367,3 +369,8 @@ warn_and_find_path(File, Dir) ->
                     []
             end
     end.
+
+format_error({cannot_read_file, Source, Reason}) ->
+    lists:flatten(io_lib:format("Cannot read file '~s': ~s", [Source, Reason]));
+format_error(Other) ->
+    io_lib:format("~p", [Other]).