exceptions: add reason to 'UsageError' exception

Use the `reason` attribute to be able to handle `UsageError` subclasses
more uniformly and simplify logic in `brew.rb` to handle them together.

Library/Homebrew/exceptions.rb

-class UsageError < RuntimeError; end
-class FormulaUnspecifiedError < UsageError; end
-class KegUnspecifiedError < UsageError; end
+class UsageError < RuntimeError
+  attr_reader :reason
+
+  def initialize(reason = nil)
+    @reason = reason
+  end
+
+  def to_s
+    s = "Invalid usage"
+    s += ": #{reason}" if reason
+    s
+  end
+end
+
+class FormulaUnspecifiedError < UsageError
+  def initialize
+    super "This command requires a formula argument"
+  end
+end
+
+class KegUnspecifiedError < UsageError
+  def initialize
+    super "This command requires a keg argument"
+  end
+end
 
 class MultipleVersionsInstalledError < RuntimeError
   attr_reader :name

Library/brew.rb

@@ -113,13 +113,9 @@ begin
     end
   end
 
-rescue FormulaUnspecifiedError
-  abort "This command requires a formula argument"
-rescue KegUnspecifiedError
-  abort "This command requires a keg argument"
-rescue UsageError
+rescue UsageError => e
   require "cmd/help"
-  Homebrew.help cmd, :usage_error => "Invalid usage"
+  Homebrew.help cmd, :usage_error => e.message
 rescue SystemExit => e
   onoe "Kernel.exit" if ARGV.verbose? && !e.success?
   $stderr.puts e.backtrace if ARGV.debug?