Incorrect error when not connected to the internet

My internet connection dropped, and suddenly my MythX credentials became invalid. I don't think this is an accurate error message.

Raise an accurate error when the computer is not connected to the Internet.

```
━━┫ truffle run verify Vulnerable2 --timeout 1
Vulnerable2 |**************************************| 100% || Elapsed: 20.0s ✗ error
These smart contracts were not found: Vulnerable2
Internal MythX errors encountered:
Invalid MythX credentials for ethereum address 0xf3....FF given.
```

Start by reproducing the `truffle run verify Vulnerable2 --timeout 1` failure with the computer disconnected, then trace the MythX request and error handling. Done means an offline connection produces an accurate connectivity error instead of reporting invalid MythX credentials.

Written by the indexing model from the issue text.