Error handling

Hi @annacruz, I've made a couple of (hopefully) improvements regarding error handling.

Firstly, I've just noted the relevant exception class in the README. This makes it quicker for developers to know which exception to rescue, and it also prompts them to write that rescue block!

Secondly, I've changed the rescues in the parser to only rescue StandardError (this is the default when no class is provided). This is a much safer pattern, I've [written about this before][post] if you're interested.

[post]: http://daniel.fone.net.nz/blog/2013/05/28/why-you-should-never-rescue-exception-in-ruby/