Do not crash command loading if unable to read enabled plugins file.

Some distributions set no permissions to read the enabled plugins
file location. This can cause a crash when running not as root or
rabbitmq user.
It was decided that plugins commands can be skipped when printing
the help message if the user have no access to the enabled plugins
file.
The warning message will be displayed in that case.

Addresses https://github.com/rabbitmq/rabbitmq-server/issues/1296
[#149079123]

deps/rabbitmq_cli/config/config.exs

@@ -34,7 +34,7 @@ use Mix.Config
 #
 # Or configure a 3rd-party app:
 #
-config :logger, level: :warn
+config :logger, [level: :warn, console: [device: :standard_error]]
 #
 
 # It is also possible to import configuration files, relative to this

deps/rabbitmq_cli/lib/rabbitmq/cli/core/command_modules.ex

@@ -61,9 +61,22 @@ defmodule RabbitMQ.CLI.Core.CommandModules do
 
   def plugin_modules(opts) do
     Helpers.require_rabbit(opts)
+    enabled_plugins =
+      try do
+        PluginsHelpers.read_enabled(opts)
+      catch err ->
+        {:ok, enabled_plugins_file} = PluginsHelpers.enabled_plugins_file(opts)
+        require Logger
+        Logger.warn("Unable to read enebled plugins file.\n" <>
+                    "  Reason: #{inspect(err)}\n" <>
+                    "  Plugins commands will not be available.\n" <>
+                    "  Please make sure you have access to\n" <>
+                    "    #{enabled_plugins_file}")
+        []
+      end
 
     partitioned =
-      Enum.group_by(PluginsHelpers.read_enabled(opts), fn(app) ->
+      Enum.group_by(enabled_plugins, fn(app) ->
         case Application.load(app) do
           :ok -> :loaded;
           {:error, {:already_loaded, ^app}} -> :loaded;