DumpTextHandler now observes DidFinishNavigation and exits 6 with empty stdout when the latest primary-frame navigation was a chromium error page or had a non-OK net::Error code. Previously, unreachable URLs exited 0 with empty stdout because chromium loads its internal error page as a real document and our handler extracted its empty body.

Verified against runtime-f6d15927b1241217:
  - https://example.com → exit 0, 130 bytes (regression check)
  - http://10.255.255.1:1/never → exit 6, empty stdout, logs net::ERR_UNSAFE_PORT
  - https://invalid.invalid/ → exit 6, empty stdout, logs net::ERR_NAME_NOT_RESOLVED

Known cosmetic: stderr log says net::net::ERR_* (double net:: prefix because ErrorToString already prefixes). Trivial fix; not worth a rebuild cycle.

HTTP error status codes (404, 500) are NOT treated as nav failures — those are real HTML served by the server.

Closes fathyb#91
Refs fathyb#88