Improve error messages on invalid URLs

In https://github.com/wojtekmach/req/issues/81, we noticed Finch exits on an invalid URL:

Mix.install([
  {:finch, "~> 0.11.0"}
])

{:ok, _} = Finch.start_link(name: MyFinch)
Finch.build(:get, "http://") |> Finch.request(MyFinch) |> IO.inspect()

** (exit) :badarg
    (finch 0.11.0) lib/finch/http1/pool.ex:62: Finch.HTTP1.Pool.request/5
    (finch 0.11.0) lib/finch.ex:294: Finch.request/3
    bug.exs:6: (file)

I did a quick survey of a couple of similar edge cases and here are the result:

defmodule Main do
  def main do
    {:ok, _} = Finch.start_link(name: MyFinch)

    get("bad")
    get("http:/")
    get("http://")
    get("https://")
  end

  def get(url) do
    Finch.build(:get, url)
    |> Finch.request(MyFinch)
    |> IO.inspect(label: inspect(url))
  catch
    kind, reason ->
      {kind, reason}
      |> IO.inspect(label: inspect(url))
  end
end

"bad": {:error, %ArgumentError{message: "scheme is required for url: bad"}}
"http:/": {:error,
 %ArgumentError{
   message: "the :hostname option is required when address is not a binary"
 }}
"http://": {:exit, :badarg}
"https://": {:error,
 %Mint.TransportError{
   reason: {:options,
    {:socket_options,
     [
       nodelay: true,
       keepalive: true,
       packet_size: 0,
       packet: 0,
       header: 0,
       active: false,
       mode: :binary
     ]}}
 }}

(The "https://" one is especially odd!)

All these errors are slightly different, perhaps it would be worthwhile to make them a bit more uniform.