Closes #3309.

Each provider in `get_external_ip` was wrapped in
`except ExternalIPNotFound`, but none of the underlying calls
(`requests.get`, `ip_to_int`, `os.popen`+`readline`,
`urllib.request.urlopen`, `json.loads`, wikipedia header lookup)
ever raise `ExternalIPNotFound`. As a result the first failing
provider crashed the entire function instead of falling through
to the next one.

Catch the exceptions that can actually occur:
  * RequestException  — for `requests.get` failures
  * AddrFormatError   — for malformed IPs
  * OSError           — for `os.popen`/curl failures
  * URLError          — for urllib failures
  * ValueError        — for json + int parse failures
  * KeyError          — for the wikipedia header
  * AssertionError    — for the isinstance guards
  * ExternalIPNotFound — kept for backward compat

Three regression tests cover: AWS connection error → curl
fallthrough, malformed AWS IP → curl fallthrough, and
all-providers-exhausted → ExternalIPNotFound.