Security: stack traces returned in API

[knisbet]

While gravity is OSS software, so returning stack traces doesn't tend to be a real information disclosure, it does provide some opportunity to observe the code paths or possibly the version of gravity in use. These aren't used, so we should disable them.

Example:

{
"error": {
"message": "access denied"
},
"traces": [
{
"path": "/gopath/src/github.com/gravitational/gravity/vendor/github.c
om/gravitational/teleport/lib/httplib/httplib.go",
"func": "github.com/gravitational/gravity/vendor/github.com/gravitati
onal/teleport/lib/httplib.WithCSRFProtection.func1",
"line": 85
},
{
"path": "/gopath/src/github.com/gravitational/gravity/vendor/github.c
om/julienschmidt/httprouter/router.go",
"func": "github.com/gravitational/gravity/vendor/github.com/juliensch
midt/httprouter.(*Router).ServeHTTP",
"line": 299
},

[knisbet]

To reproduce: curl -k -H "Content-Type: application/json" -d '{"username": "example", "password":"invalid"}' https://<web server>/proxy/v1/webapi/sessions