Return :ok/:error tuples by default

[teamon]

{:ok, env} - after successful HTTP transaction (with status, headers, body)
{:error, reason} - all connection errors, nxdomain, connrefused etc

• Change the return type to {:ok, Tesla.Env.t} | {:error, error}
• Update README
• Update all docs

[teamon]

If you're a brave one you can try the 1.0-tuples branch now ;)

[teamon]

Merged into 1.0 branch