Leave kegs keg-only if linking step fails

Rationale: well, it should always have been like this!

However now we are opening ourselves up to more-mixed installations of formula not maintained by us, it's important that

Library/Homebrew/cmd/link.rb

@@ -11,12 +11,7 @@ module Homebrew extend self
     ARGV.kegs.each do |keg|
       print "Linking #{keg}... "
       puts if ARGV.verbose?
-      begin
       puts "#{keg.link} symlinks created"
-      rescue Exception
-        puts
-        raise
-      end
     end
   end
 end

Library/Homebrew/extend/pathname.rb

@@ -273,15 +273,10 @@ class Pathname
 
     self.dirname.mkpath
     Dir.chdir self.dirname do
-      # TODO use Ruby function so we get exceptions
-      # NOTE Ruby functions may work, but I had a lot of problems
-      rv = system 'ln', '-sf', src.relative_path_from(self.dirname), self.basename
-      unless rv and $? == 0
-        raise <<-EOS.undent
-          Could not create symlink #{to_s}.
-          Check that you have permissions on #{self.dirname}
-          EOS
-      end
+      # NOTE only system ln -s will create RELATIVE symlinks
+      system 'ln', '-s', src.relative_path_from(self.dirname), self.basename
+      # ln outputs useful error message for us
+      raise "Could not create symlink: #{to_s}." unless $?.success?
     end
   end
 

Library/Homebrew/formula_installer.rb

@@ -214,11 +214,14 @@ class FormulaInstaller
       f.linked_keg.unlink
     end
 
-    Keg.new(f.prefix).link
+    keg = Keg.new(f.prefix)
+    keg.link
   rescue Exception => e
     onoe "The linking step did not complete successfully"
     puts "The formula built, but is not symlinked into #{HOMEBREW_PREFIX}"
     puts "You can try again using `brew link #{f.name}'"
+    keg.unlink
+
     ohai e, e.backtrace if ARGV.debug?
     @show_summary_heading = true
   end