Add `error.include-stacktrace` configuration option to determine when
stack trace attributes should be added. Options are `never`, `always`
or `on-trace-param`.

Prior to this commit stacktrace attributes were never includes in HTML
responses, and only included in JSON responses when a `trace` request
parameter was set to `true`. This has now been changed so that,
by default, stacktrace attributes are never included.

The BasicErrorController includes a protected method that can be
overridden if includes should be different depending on the `produces`
mapping.

Closes gh-2371