fix(playground): avoid stringifying invalid console messages

[caugner]

Summary

(MP-1063)

Follow-up to #10962.

Problem

Invalid console messages can crash the Playground.

Solution

Avoid stringifying invalid console messages.

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How did you test this change?

1. Ran yarn && yarn dev and opened http://localhost:3000/en-US/play
2. Pasted in the following JavaScript:

   var o = {};
   o.x = o;
   parent.postMessage({typ:'console',prop:'log',message:o}, '*');

3. Verified that the Playground doesn't crash and the Playground console shows:

   [Playground] Unsupported console message (see browser console)