Specific error class for missing attribute accesors.

[tomasmiguez]

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Hi there, we are currently having an issue with the initialization logic. As a send is used with the passed attributes, when some received attribute is not declared as an attribute accesor, a NoMethodError is raised. This used to be fine, but as the application in which wi use Scrivener grew, this errors became more common and started masking really critical NoMethodErrors. It would be great if a configuration could be introduced to raise a custom Scrivener exception instead of the generic one. I wouldn't mind iterating a PR if a mantainer validates this approach.