Add dataset validation for sync push command

[DechengMa]

Adding validation and error handling logic for sync push command

📝 Summary of Changes

Changes proposed in this pull request:

• Add duplicateTesters error for handling duplicate testers with the same email addresses.
• Add noValidApp error when the user hasn't run sync pull before sync push

🧐🗒 Reviewer Notes

💁 Example

swift run asc testflight sync push

🔨 How To Test

swift run asc testflight sync push

[adamjcampbell]

There's an argument to be made around not adding equatable conformance in the implementation if you only use it for testing. Not sure how I feel about it though

[DechengMa]

There's an argument to be made around not adding equatable conformance in the implementation if you only use it for testing. Not sure how I feel about it though

Any ideas for removing the Equatable in this case? 🤔

[adamjcampbell]

If you wanted to you could replace the instances in testing with something like:

XCTAssertThrowsError(try TestFlightProgramDifference(local: local, remote: remote)) { error in
  guard case TestFlightProgramDifference.Error.duplicateTesters(let email) = error else {
    return XCTFail()
  }

  XCTAssertEqual(email, "foo@gmail.com")
}