More specific exception

[owenRiddy]

The Quickstart proved misleading (https://github.com/cgrand/enlive/issues/154) and didn't adequately signal that the word resource meant a Java resource. It looks a lot like a file path.

This PR improves the exception thrown from NullPointerException to MissingResourceException and adds a hint to the error message with more context.

it assumes that people are more likely to run into this failure mode when they follow the Quickstart int he README.