repl: do not crash on async op reject

[kevinkassimo]

Adds a custom error handler that only prints instead of exiting the process when an async error is thrown (which cannot be caught by repl loop).

Before:

> Deno.open()
Promise {}
> error: Uncaught InvalidData: missing field `filename`
► $deno$/dispatch_json.ts:40:11
    at DenoError ($deno$/errors.ts:20:5)
    at unwrapResponse ($deno$/dispatch_json.ts:40:11)
    at sendAsync ($deno$/dispatch_json.ts:91:10)

(Process crashes, terminal mode is not properly set and keys pressed are not echoed back to terminal)

After:

> Deno.open()
Promise {}
> error: Uncaught InvalidData: missing field `filename`
► $deno$/dispatch_json.ts:40:11
    at DenoError ($deno$/errors.ts:20:5)
    at unwrapResponse ($deno$/dispatch_json.ts:40:11)
    at sendAsync ($deno$/dispatch_json.ts:91:10)
> console.log("not crashed")
not crashed
undefined
>

[ry]

LGTM - thanks!