Use ConnectError.unavailable when client is offline

[rebello95]

Resolves #325.

When turning the device's wifi off, the following was observed with URLSession:

Before:

▿ Optional<ConnectError>
  ▿ some : ConnectError
    - code : Connect.Code.unknown
    ▿ message : Optional<String>
      - some : "The Internet connection appears to be offline."
    ▿ exception : Optional<Error>
      - some : Error Domain=NSURLErrorDomain Code=-1009 "The Internet connection appears to be offline." 

Based on the discussion in the issue linked above, this PR updates the code to be .unavailable:

▿ Optional<ConnectError>
  ▿ some : ConnectError
    - code : Connect.Code.unavailable
    ▿ message : Optional<String>
      - some : "The Internet connection appears to be offline."
    ▿ exception : Optional<Error>
      - some : Error Domain=NSURLErrorDomain Code=-1009 "The Internet connection appears to be offline." 

The NIO client has also been updated to return the .unavailable status code:

▿ Optional<ConnectError>
  ▿ some : ConnectError
    - code : Connect.Code.unavailable
    ▿ message : Optional<String>
      - some : "client is not connected"
    - exception : nil

[aheze]

Btw does this work for NIOHttpClient?

[rebello95]

Btw does this work for NIOHttpClient?

I just updated the PR to handle both NIO and URLSession 👍🏽