Better error handling

[vltr]

Provide better error handling techniques for developers (almost everywhere)

[ahopkins]

Perhaps allowing for capturing all validation errors and not just the first?

[vltr]

@ahopkins yes, this is a limitation for using attrs to generate the classes boilerplate code. Basically, it's impossible to have a fine grained control over either to validate one single class or not, you either have everything being validated (which is good only for production - no need to waste resources if your data is somehow wrong) or nothing is validated, based on a global variable - that can lead to racing condition problems.