(#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