error handling for classpath schema client

[rjruizes]

Currently, if you reference a schema file that does not exist, you get this error message:

unknown protocol: classpath

because the classpath URL is being passed to the fallback client, which can't process it.

This PR improves the error message to name the classpath resource that cannot be found:

Caused by: java.io.IOException: Could not find classpath://com/test/predefined.json

[coveralls]

Coverage increased (+0.01%) to 91.54% when pulling e669256 on rjruizes:master into 721c1bc on everit-org:master.

[erosb]

Nice fix, thank you for your work @rjruizes !