feat(build): fail build if xref complains

[janl]

only works on Erlang 23. 24 and 25 fail

24 e.g. with:

Warning: hastings_index:await/2 is undefined function (Xref)
Warning: hastings_index:design_doc_to_indexes/1 is undefined function (Xref)
Warning: hastings_index_manager:get_index/2 is undefined function (Xref)
WARN:  Continuing on after abort: [xref]
ERROR: xref failed while processing /Users/jan/Work/asf/tmp/couchdb/src/mem3: {'EXIT',
 {{badmatch,
   [{attribute,{1,1},file,{"src/mem3_reshard_index.erl",1}},
    {attribute,{13,2},module,mem3_reshard_index},
    {attribute,
     {15,2},
     export,
     [{design_docs,1},{target_indices,2},{spawn_builders,1},{build_index,2}]},
… big symbol dump