fix NoMethodError: undefined method `zero?' for nil:NilClass

[equivalent]

we had misconfiguration on Rails application on our server that was incorrectly compiling assets.

this was cousing Airbreak to capture non descriptive error

NoMethodError: undefined method `zero?' for nil:NilClass

File    /gems/yui-compressor-0.12.0/lib/yui/compressor.rb:101
Hostname    c0e5a07008d2
Error type  NoMethodError
Error message   NoMethodError: undefined method `zero?' for nil:NilClass
Where   rake#assets:precompile

there is alternative PR version of this here #46

[equivalent]

with this implemented, output in case of error will look like this:

rake assets:precompile
YUI::Compressor::RuntimeError: Command 'java -jar /usr/local/lib/ruby/gems/2.2.0/gems/yui-compressor-0.12.0/lib/yui/../yuicompressor-2.4.8.jar --type css --charset utf-8 /tmp/yui_compress20160113-164-xb6596' returned non-zero exit status