improve error on no index.html

[c-bik]

2019-08-20 09:41:51.311 [error] [<0.979.0>@dderl:init:104] CRASH REPORT
Process <0.979.0> with 0 neighbours crashed with reason: no match of right hand value
 {error,enoent} in dderl:init/2 line 104
2019-08-20 09:41:51.314 [error]  Lager event handler error_logger_lager_h exited with reason
 {'EXIT',{{case_clause,[https,<0.978.0>,1,<0.979.0>,{badmatch,{error,enoent}},
[{dderl,init,2,[{file,"dderl/_build/default/lib/dderl/src/dderl.erl"},{line,104}]},
{cowboy_handler,execute,2,[{file,"dderl/_build/default/lib/cowboy/src/cowboy_handler.erl"},{line,37}]},
{cowboy_stream_h,execute,3,[{file,"dderl/_build/default/lib/cowboy/src/cowboy_stream_h.erl"},{line,274}]},
{cowboy_stream_h,request_process,...},...]]},...}}