Catch the message of VariableDuplicationError

[ksss]

ref: ruby/rbs#2310

RBS::InstanceVariableDuplicationError and RBS::ClassInstanceVariableDuplicationError have been introduced in rbs, but since Steep does not recognize these errors, it results in an Unexpected error.

This PR fixes the issue so that Steep can properly display validation messages for these errors.

Message example

variable-duplication.rbs:3:2: [error] Duplicated instance variable name `@i` in `::Instance`
│ Diagnostic ID: RBS::InstanceVariableDuplicationError
│
└   @i: Integer
    ~~~~~~~~~~~

variable-duplication.rbs:8:2: [error] Duplicated class instance variable name `@ci` in `::ClassInstance`
│ Diagnostic ID: RBS::ClassInstanceVariableDuplicationError
│
└   self.@ci: Integer
    ~~~~~~~~~~~~~~~~~

[soutaro]

🙏