Better error handling for inaccessible DNS servers

[celskeggs]

When we specify the wrong DNS servers, we get an error from the debian installer that looks like the following:

[            (1*installer)  2 shell  3 shell  4- log           ][ Feb 15 18:30 ]

   ┌────────────┤ [!!] Choose a mirror of the Debian archive ├─────────────┐
   │                                                                       │
   │                          Bad archive mirror                           │
   │ An error has been detected while trying to use the specified Debian   │
   │ archive mirror.                                                       │
   │                                                                       │
   │ Possible reasons for the error are: incorrect mirror specified;       │
   │ mirror is not available (possibly due to an unreliable network        │
   │ connection); mirror is broken (for example because an invalid Release │
   │ file was found); mirror does not support the correct Debian version.  │
   │                                                                       │
   │ Additional details may be available in /var/log/syslog or on virtual  │
   │ console 4.                                                            │
   │                                                                       │
   │ Please check the specified mirror or try a different one.             │
   │                                                                       │
   │     <Go Back>                                          <Continue>     |
   │                                                                       │
   └───────────────────────────────────────────────────────────────────────┘


<Tab> moves; <Space> selects; <Enter> activates buttons

This is not an explanatory error, and you have to be manually tailing log.jet.install in order to see it during an autoinstall. We should provide a useful error message in this situation, both directly in the installer and in the user's autoinstall output.