(#15307) Fix puppet help when a face throws an exception

Without this patch applied the `puppet help` command displays nothing
but an exception when a 3rd party face throws an exception during the
loading process.

For example:

    Error: Could not autoload puppet/face/node/classify: no such file to load -- fog
    Error: Could not autoload puppet/face/node: Could not autoload puppet/face/node/classify: no such file to load -- fog
    Error: Failed to load face node:
    Error: Could not find version current of node
    Error: Try 'puppet help help help' for usage

This is a problem because `puppet help` should be robust and provide helpful
information in all cases.

This patch fixes the problem by catching loading errors when building the list
of available face subcommands and their summaries.  If an exception is caught,
it is handled by marking that face subcommand and presenting a message
indicating the face is unavailable in the help output.

This looks like:

    Usage: puppet <subcommand> [options] <action> [options]

    Available subcommands:

      agent             The puppet agent daemon
      apply             Apply Puppet manifests locally
      ca                Local Puppet Certificate Authority management.
      catalog           Compile, save, view, and convert catalogs.
      cert              Manage certificates and requests
      certificate       Provide access to the CA for certificate management.
      certificate_request  Manage certificate requests.
      certificate_revocation_list  Manage the list of revoked certificates.
      config            Interact with Puppet's configuration options.
      describe          Display help about resource types
      device            Manage remote network devices
      doc               Generate Puppet documentation and references
      facts             Retrieve and store facts.
      file              Retrieve and store files in a filebucket
      filebucket        Store and retrieve files in a filebucket
      help              Display Puppet help.
      inspect           Send an inspection report
      instrumentation_data  Manage instrumentation listener accumulated data.
      instrumentation_listener  Manage instrumentation listeners.
      instrumentation_probe  Manage instrumentation probes.
      key               Create, save, and remove certificate keys.
      kick              Remotely control puppet agent
      man               Display Puppet manual pages.
      master            The puppet master daemon
      module            Creates, installs and searches for modules on the Puppet Forge.
      ! node            ! Subcommand unavailable due to error. Check error logs.
      ! node_aws        ! Subcommand unavailable due to error. Check error logs.
      ! node_vmware     ! Subcommand unavailable due to error. Check error logs.
      parser            Interact directly with the parser.
      plugin            Interact with the Puppet plugin system.
      queue             Queuing daemon for asynchronous storeconfigs
      report            Create, display, and submit reports.
      resource          The resource abstraction layer shell
      resource_type     View classes, defined resource types, and nodes from all manifests.
      secret_agent      Mimics puppet agent.
      status            View puppet server status.

    See 'puppet help <subcommand> <action>' for help on a specific subcommand action.
    See 'puppet help <subcommand>' for help on a specific subcommand.
    Puppet v3.0.0