Error messages updates

[gworkman]

Noticed a few rough edges while deploying a firmware today:

• nerves_hub help throws an error message. It currently expects nerves_hub --help
• likewise, nerves_hub help COMMAND also throws an error message
• nerves_hub firmware sign help text should be nerves_hub firmware sign /PATH/TO/FIRMWARE --key KEYNAME. (and best case is we can detect the firmware file rather than having to explicitly pass it it)
• nerves_hub product delete PRODUCT_NAME doesn't handle success correctly: Unhandled error: {:ok, nil}
• nerves_hub product create doesn't accept the default (directory name):

❯ nerves_hub product create
NervesHub server: manage.nervescloud.com:443
NervesHub organization: protolux
Product name (default soleil_demo):

Creating product ''...
Unhandled error: {:error, %{"errors" => %{"name" => ["can't be blank"]}}}

[joevandyk]

Relatedly, getting ** (ArgumentError) errors were found at the given arguments: * 1st argument: not a binary when trying to list firmware - i'm assuming that it's because something's not configured right, but it should probably give a better error message?

➜  print_server git:(jvd/cleanups) ✗ mix nerves_hub.firmware list
==> nerves
==> print_server_rpi3
Generated print_server_rpi3 app
==> print_server

Nerves environment
  MIX_TARGET:   print_server_rpi3
  MIX_ENV:      dev

==> nerves_hub_cli
Compiling 45 files (.ex)
     warning: function config/0 is unused
     │
 204 │   defp config() do
     │        ~
     │
     └─ lib/nerves_hub_cli/cli/utils.ex:204:8: NervesHubCLI.CLI.Utils (module)

Generated nerves_hub_cli app
==> print_server
Generated print_server app
NervesHub server: <server>:443
NervesHub organization: crowdcow
** (ArgumentError) errors were found at the given arguments:

  * 1st argument: not a binary

    :erlang.binary_to_atom(nil)
    (tesla 1.12.3) lib/tesla/adapter/mint.ex:161: Tesla.Adapter.Mint.open_conn/2
    (tesla 1.12.3) lib/tesla/adapter/mint.ex:121: Tesla.Adapter.Mint.do_request/5
    (tesla 1.12.3) lib/tesla/adapter/mint.ex:61: Tesla.Adapter.Mint.call/2
    (tesla 1.12.3) lib/tesla/middleware/json.ex:57: Tesla.Middleware.JSON.call/3
    (tesla 1.12.3) lib/tesla/middleware/follow_redirects.ex:46: Tesla.Middleware.FollowRedirects.redirect/3
    (nerves_hub_cli 2.1.0) lib/nerves_hub_cli/api.ex:54: NervesHubCLI.API.request/4
    (nerves_hub_cli 2.1.0) lib/mix/tasks/nerves_hub.firmware.ex:118: Mix.Tasks.NervesHub.Firmware.list/2