Add better Unix exit codes.

Closes Homebrew/homebrew#10809.

Library/Homebrew/cmd/doctor.rb

@@ -836,5 +836,6 @@ module Homebrew extend self
     end
 
     puts "Your system is raring to brew." if raring_to_brew
+    exit raring_to_brew ? 0 : 1
   end
 end

Library/Homebrew/cmd/install.rb

@@ -87,6 +87,7 @@ module Homebrew extend self
           fi.finish
         rescue CannotInstallFormulaError => e
           onoe e.message
+          exit 1
         end
       end
     end

Library/Homebrew/cmd/upgrade.rb

@@ -66,9 +66,11 @@ module Homebrew extend self
     installer.finish
   rescue CannotInstallFormulaError => e
     onoe e
+    exit 1
   rescue BuildError => e
     e.dump
     puts
+    exit 1
   ensure
     # restore previous installation state if build failed
     outdated_keg.link if outdated_keg and not f.installed? rescue nil