Add common exception class

[vladcostea]

• Allows clients to rescue the common ServiceError class if they don't
  care what error the cmis service raised

[bertBruynooghe]

How about ordering them alpabetically? Would make things more maintainable...

[bertBruynooghe]

Get rid of the exceptions module, it just adds complexity.

[vladcostea]

I'd rather not, as this will surely break backwards compatibility with certain clients of this lib that rely on rescuing these exceptions.

Also, by removing the Exceptions namespace, assuming we still want to enforce a common parent class for all CMIS service related exceptions/errors, we would have to write CMIS::StorageException iso CMIS::Exceptions::Storage, which to me, is the same thing.

If you feel that strongly about removing the Exceptions namespace a compromise would be to introduce a new exception parent for each exception that sits between ServiceError and the exception.

For example

module CMIS
  ServiceError = Class.new(StandardError)
  StorageException = Class.new(ServiceError)
  module Exceptions
    Storage = Class.new(StorageException)
    ...