Fix representation of simple object errors (currently "[object Object]")

[GlennKintscher]

Description

If a library throws a simple object as an error, it is just displayed as "[object Object]", which is not very helpful for troubleshooting. This fix changes the behavior so that errors are converted to a JSON string if they are not of type Error.

Example

Location	Current behaviour	Expected behaviour
Console
Online

[mattdyoung]

Hi @GlennKintscher

Thanks for the PR! This is a good idea which we'd like to take forward, but we have to be careful about trying to JSON stringify things as it can cause errors.

The readme for safe-json-stringify, which we maintain a fork of is quite informative https://github.com/debitoor/safe-json-stringify, so if converting to JSON we should ensure we use our safe-json-stringify module.

Alternatively, there is prior art in the repo for doing this with arbitrary args to for leaving console log arguments as breadcrumbs

bugsnag-js/packages/plugin-console-breadcrumbs/console-breadcrumbs.js

Lines 18 to 26 in 21a1b08

	let stringified = '[Unknown value]'
	// this may fail if the input is:
	// - an object whose [[Prototype]] is null (no toString)
	// - an object with a broken toString or @@toPrimitive implementation
	try { stringified = String(arg) } catch (e) {}
	// if it stringifies to [object Object] attempt to JSON stringify
	if (stringified === '[object Object]') {
	// catch stringify errors and fallback to [object Object]
	try { stringified = JSON.stringify(arg) } catch (e) {}

so we could factor that out and use it in both places.

We'll review this and either suggest the necessary changes or more likely factor this in to our own PR.