Fix error reporting formatting

[multani]

The stacktrace for an error was formatted as:

${message}
${stacktrace}

I'm pretty sure I saw that documented in Google Cloud's documentation when I initially implemented this feature, but I can't find it anymore and the current documentation from
https://cloud.google.com/error-reporting/docs/formatting-error-messages#log-error now says:

To log an error event that is a stack trace, write the error event as
one of these types:
[...]

• A jsonPayload that includes a message, stack_trace, or
  exception field.

  You can specify more than one of those fields. If more than one of
  those fields is specified, then the order of evaluation is: stack_trace,
  then exception, and then message.

  If the message field is evaluated and if it isn't empty, then the
  stack trace is captured only when the field contains a stack trace in
  one of the supported programming language formats. The stack trace isn't
  captured by Error Reporting when an unsupported format is used.

  If your error event is formatted as a ReportedErrorEvent object,
  then copy its fields to the jsonPayload. For more information and an
  example, see Log an error that is formatted as a ReportedErrorEvent
  object.

There's no mention of this weird ${message}\n${stacktrace} format anymore, so let's get rid of it.

Fix: #96