Move 10.11 warning to doctor, pre-install, and post-failure

Fixes Homebrew/homebrew#40778.

Library/Homebrew/cmd/doctor.rb

@@ -237,6 +237,15 @@ def check_for_broken_symlinks
   end
 end
 
+def check_for_unsupported_osx
+  if MacOS.version >= "10.11" then <<-EOS.undent
+    You are using OS X #{MacOS.version}.
+    We do not provide support for this pre-release version.
+    You may encounter build failures or other breakage.
+    EOS
+  end
+end
+
 if MacOS.version >= "10.9"
   def check_for_installed_developer_tools
     unless MacOS::Xcode.installed? || MacOS::CLT.installed? then <<-EOS.undent

Library/Homebrew/cmd/install.rb

@@ -112,6 +112,7 @@ module Homebrew
   def check_xcode
     checks = Checks.new
     %w[
+      check_for_unsupported_osx
       check_for_installed_developer_tools
       check_xcode_license_approved
       check_for_osx_gcc_installer

Library/Homebrew/exceptions.rb

@@ -216,6 +216,11 @@ class BuildError < RuntimeError
       puts "These open issues may also help:"
       puts issues.map{ |i| "#{i['title']} (#{i['html_url']})" }.join("\n")
     end
+
+    if MacOS.version >= "10.11"
+      require "cmd/doctor"
+      opoo Checks.new.check_for_unsupported_osx
+    end
   end
 end
 

Library/brew.rb

@@ -50,15 +50,6 @@ if OS.mac? and MacOS.version < "10.6"
   EOABORT
 end
 
-if OS.mac? && MacOS.version == "10.11"
-  opoo <<-EOS.undent
-    Although Homebrew has added initial recognition of 10.11, it is unsupported.
-    You may encounter breakage or other failure and there is no guarantee
-    Homebrew can resolve those issues until El Capitan is stable.
-
-  EOS
-end
-
 # Many Pathname operations use getwd when they shouldn't, and then throw
 # odd exceptions. Reduce our support burden by showing a user-friendly error.
 Dir.getwd rescue abort "The current working directory doesn't exist, cannot proceed."