swift-package-migrate: Emit feature validation errors through the diagnostic emitter

These errors do not represent issues with parsing command-line
arguments, so throwing a `ValidationError` is not suitable — we don't
want to present the user with a command usage hint in these cases.

Author: AnthonyLatsis

Sources/Commands/PackageCommands/Migrate.swift

@@ -65,7 +65,9 @@ extension SwiftPackageCommand {
 
         public func run(_ swiftCommandState: SwiftCommandState) async throws {
             // First, validate and resolve the requested feature names.
-            let features = try self.resolveRequestedFeatures(swiftCommandState)
+            guard let features = try self.resolveRequestedFeatures(swiftCommandState) else {
+                return
+            }
 
             let buildSystem = try await createBuildSystem(
                 swiftCommandState,
@@ -166,9 +168,12 @@ extension SwiftPackageCommand {
         }
 
         /// Resolves the requested feature names.
+        ///
+        /// - Returns: An array of resolved features, or `nil` if an error was
+        ///   emitted.
         private func resolveRequestedFeatures(
             _ swiftCommandState: SwiftCommandState
-        ) throws -> [SwiftCompilerFeature] {
+        ) throws -> [SwiftCompilerFeature]? {
             let toolchain = try swiftCommandState.productsBuildParameters.toolchain
 
             // Query the compiler for supported features.
@@ -188,13 +193,15 @@ extension SwiftPackageCommand {
                         .sorted()
                         .joined(separator: ", ")
 
-                    throw ValidationError(
-                        "Unsupported feature '\(name)'. Available features: \(migratableCommaSeparatedFeatures)"
+                    swiftCommandState.observabilityScope.emit(
+                        error: "Unsupported feature '\(name)'. Available features: \(migratableCommaSeparatedFeatures)"
                     )
+                    return nil
                 }
 
                 guard feature.migratable else {
-                    throw ValidationError("Feature '\(name)' is not migratable")
+                    swiftCommandState.observabilityScope.emit(error: "Feature '\(name)' is not migratable")
+                    return nil
                 }
 
                 resolvedFeatures.append(feature)