write out cache file when raising a build error exception

lib/rscons/environment.rb

@@ -149,6 +149,7 @@ module Rscons
       @targets.each do |target, info|
         next if targets_processed.include?(target)
         unless process_target.call(target)
+          cache.write
           raise BuildError.new("Failed to build #{target}")
         end
       end

lib/rscons/version.rb

@@ -1,4 +1,4 @@
 module Rscons
   # gem version
-  VERSION = "0.0.4"
+  VERSION = "0.0.5"
 end