wm_timeseries: cumulative commit to fix the following:

* accurately check and report missing/extra/unpaired field/value pairs
  in get/delete URLs;
* avoid raw Erlang terms in error messages;
* in particular, make lexer/parser error readable again;
* uniformly report error messages as text/plain;
* re-enable check for api_version;
* correctly check URL parameters (like ?timeout=5);
* correctly check for service availability (use init:get_status/0);
* use spaces around '=' etc, and reindent.