use the classname as message in case the exception has no message

fixes https://github.com/jruby/jruby/issues/2249

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use JRubyFile to get input-stream to file-resource fixes #11

this is just a mix of the old code, the new code and what the ChannelDescriptor
of jruby-1.7.x is doing. but works only for jruby-1.7.14 or newer with a fallback
to new code.

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