debugger: fix protocol debug log

The commit 721404 modified internal debugger implementation to use
Node's debugger `Protocol` to parse data sent by the V8 debugger.
With that change in place, `msg.body` is no longer a string, but
a (JSON) object instead. As a result, `event` and `response` entries
printed `[object Object]` instead of the JSON payload.

This commit fixes the problem by calling `JSON.stringify(msg.body)`.
BreakEventHandler: fix break on uncaught exception

Fix a bug where `TypeError: undefined is not a function` was ignored
in "break on uncaught exceptions" mode.

This exception is thrown by a V8 builtin function CALL_NON_FUNCTION
defined in an internal script `v8/src/runtime.js`, which is not
tracked by Node Inspector. Before this change, Node Inspector
incorrectly assumed that the script is a code loaded via `eval`.