write out cache file when raising a build error exception

lib/rscons.rb

@@ -17,4 +17,7 @@ module Rscons
     Object,
     Program,
   ]
+
+  class BuildError < Exception
+  end
 end

lib/rscons/environment.rb

@@ -149,8 +149,8 @@ module Rscons
       @targets.each do |target, info|
         next if targets_processed.include?(target)
         unless process_target.call(target)
-          $stderr.puts "Error: failed to build #{target}"
-          break
+          cache.write
+          raise BuildError.new("Failed to build #{target}")
         end
       end
       cache.write

lib/rscons/version.rb

@@ -1,4 +1,4 @@
 module Rscons
   # gem version
-  VERSION = "0.0.3"
+  VERSION = "0.0.5"
 end