support wget exit statuses

[mattgathu]

Wget may return one of several error codes if it encounters problems.

• [ ] 0 No problems occurred.
• [ ] 1 Generic error code.
• [ ] 2 Parse error---for instance, when parsing command-line options, the .wgetrc or .netrc...
• [ ] 3 File I/O error.
• [ ] 4 Network failure.
• [ ] 5 SSL verification failure.
• [ ] 6 Username/password authentication failure.
• [ ] 7 Protocol errors.
• [ ] 8 Server issued an error response.