Suppress backtrace errors passed over the API.

[PietrH]

When an Error is returned over the API, the backtrace is included in the HTTP400 text response. This is confusing to the user, and we should suppress this.

Strangely, the backtrace is not included when I use etnservice directly.

[PietrH]

Might be relevant: opencpu/opencpu#422

The trace might be introduced by opencpu, is there a way to suppress it? or provide a .trace_bottom (rlang::abort)