When `Format_event::format_event(...)` returns an error, we are
currently silently dropping that
Event. tokio-rs/valuable#88 explains one
such case in which this was encountered (due to a bug in
valuable-serde). We want to be made aware whenever an Event is dropped.

This patch adds a single `eprintln` line to let the user know that we
were unable to format a specific event. We are not emitting an actual
tracing Event, to avoid the risk of a cycle (the new Event could
trigger the same formatting error again).

Resolves tokio-rs#1965.

…lder

Update the error-handling logic when calling `format_event` to write
the error message to the Writer. Only if that fails do we write to
stderr.

Define new method `SubscriberBuilder::with_silence_errors(bool)` so
that a user may disable this behavior. Enabled by default.

Add unit tests for both `silence_errors=true` and `false`.

…om:bryangarza/tracing into feat-1965-stderr-for-subscriber-fmt-errors

Co-authored-by: David Barsky <me@davidbarsky.com>

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

…stic assertions

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

This patch updates `with_silence_errors` to be named
`with_silence_internal_errors`, and also updates the rustdocs so that
it's more clear what its behavior is.

This patch renames the `with_silence_internal_errors` flag to simply
`log_internal_errors`, and switches the default value to `true` since
the flag now represents the opposite idea.