inspector: no crash when WS server can't start

This change also changes error message to make it consistent with the
one printed by the debugger.

Fixes: https://github.com/nodejs/node/issues/10858
PR-URL: https://github.com/nodejs/node/pull/10878


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>