help: show command-specific help for invalid usage

Instead of always printing the generic help text, print command-specific
help if it is available and a command raised the `UsageError` exception.
Put the error message underneath the help text (was above) to avoid that
it scrolls off the screen.

Thereby fix a regression where handling the invalid usage would fail to
access `ARGV.usage` removed in c6536066dc.

Library/Homebrew/cmd/help.rb

@@ -40,6 +40,14 @@ module Homebrew
       path = command_path(cmd)
     end
 
+    # Display command-specific (or generic) help in response to `UsageError`.
+    if (error_message = flags[:usage_error])
+      $stderr.puts path ? command_help(path) : HOMEBREW_HELP
+      $stderr.puts
+      onoe error_message
+      exit 1
+    end
+
     # Handle `brew` (no arguments).
     if flags[:empty_argv]
       $stderr.puts HOMEBREW_HELP

Library/brew.rb

@@ -118,8 +118,8 @@ rescue FormulaUnspecifiedError
 rescue KegUnspecifiedError
   abort "This command requires a keg argument"
 rescue UsageError
-  onoe "Invalid usage"
-  abort ARGV.usage
+  require "cmd/help"
+  Homebrew.help cmd, :usage_error => "Invalid usage"
 rescue SystemExit => e
   onoe "Kernel.exit" if ARGV.verbose? && !e.success?
   $stderr.puts e.backtrace if ARGV.debug?