MacOS.dev_tools_path: return nil if not found

This matches the semantics of other path methods, such as Xcode.prefix.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/global.rb

@@ -99,5 +99,5 @@ ORIGINAL_PATHS = ENV['PATH'].split(':').map{ |p| Pathname.new(File.expand_path(p
 # Xcode-only installs place tools in non-standard locations, and we also want
 # to ensure the dev tools are in the PATH in build.rb
 unless ORIGINAL_PATHS.include? MacOS.dev_tools_path
-  ENV['PATH'] = ENV['PATH'].to_s + ':' + MacOS.dev_tools_path
+  ENV['PATH'] = ENV['PATH'].to_s + ':' + MacOS.dev_tools_path.to_s
 end

Library/Homebrew/macos.rb

@@ -71,8 +71,7 @@ module MacOS extend self
     else
       # Since we are pretty unrelenting in finding Xcode no matter where
       # it hides, we can now throw in the towel.
-      opoo "You really should consult the `brew doctor`!"
-      ""
+      opoo "Could not locate developer tools. Consult `brew doctor`."
     end
   end