scrooge-sbt-plugin should complain if exception identifiers are not unique

[reikje]

When you define a service function that throws multiple exceptions, but you use the same exception identifier/name, scrooge-sbt-plugin should already fail to generate code.

Expected behavior

The generate step should fail with a similar error that the vanilla thrift generator produces: "2: e" - field identifier/name has already been used.

Actual behavior

Code is generated but doesn't compile, so the user is left to figure out why. If the generate task would fail due to invalid syntax, you would immediately see what the problem is.

Steps to reproduce the behavior

Use the following thrift definition:

exception ArgumentInvalidException {
    1: string message
}

exception ServiceException {
    1: string message
}

service SomeService {
    string foo(1: string name)
        throws (1: ServiceException e, 2: ArgumentInvalidException e)
}