`raise` instead of `throw` ?

[ericproulx]

There are few places in the code that we raise an exception instead of throw :error and vice-versa.

• Grape::Middleware::Versioner::AcceptVersionHeader middleware is throwing but Grape::Middleware::Versioner::Header is raising.
• Running validators is raising.
• Both throw and exception are dealt by error_response in Grape::Middleware::Error and Grape::Exceptions::Base needs the [] operator so the code can work when its Hash or `Exception. This is really confusing.

We should be consistent in the code base since we mostly treat throw like exception. For instance, when dealing with Grape::Exceptions::ValidationErrors, we rescue them through rescue_from and throw and :error afterward with the backtrace and original_exception.