Prevent customInspect error from crashing console

[kevinkassimo]

Add a try-catch and allow fallback stringify for customInspect.

A current crashing example is inspection towards Headers.prototype. This PR fixes this:

Before:

> Headers.prototype
error: Uncaught TypeError: Cannot read property 'size' of undefined
► $deno$/headers.ts:90:38
    at [Deno.customInspect] ($deno$/headers.ts:90:38)
    at createObjectString ($deno$/console.ts:330:40)
    at stringify ($deno$/console.ts:143:14)
    at stringifyArgs ($deno$/console.ts:462:14)
    at replLog ($deno$/repl.ts:18:14)
    at evaluate ($deno$/repl.ts:89:5)
    at replLoop ($deno$/repl.ts:176:13)

After:

> Headers.prototype
HeadersBase {}

[ry]

LGTM