rgw: make init env methods return an error

rgw:  make init env methods return an error

Since web frontends may signal an error when requests are malformed or so, let
us double check this and raise errors early. The current user of this is
civetweb frontend; which can potentially return null from `parse_http_headers`
when a HTTP header without a ":" is supplied at which point headers.value is
null which can lead to undefined behaviour later in RGW.

Fixes: http://tracker.ceph.com/issues/23039
Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>