Add SwiftPolyglotError and error handling

Sources/SwiftPolyglot/main.swift

@@ -16,5 +16,6 @@ let swiftPolyglot: SwiftPolyglot = .init(
 do {
     try swiftPolyglot.run()
 } catch {
-    print(error)
+    print(error.localizedDescription)
+    exit(EXIT_FAILURE)
 }

Sources/SwiftPolyglotCore/SwiftPolyglot.swift

@@ -11,8 +11,7 @@ public struct SwiftPolyglot {
 
     public func run() throws {
         guard !arguments.isEmpty else {
-            print("Usage: script.swift <language codes> [--errorOnMissing]")
-            exit(1)
+            throw SwiftPolyglotError.noLanguageCodes
         }
 
         let isRunningFromGitHubActions = ProcessInfo.processInfo.environment["GITHUB_ACTIONS"] == "true"
@@ -79,8 +78,7 @@ public struct SwiftPolyglot {
         searchDirectory()
 
         if missingTranslations, errorOnMissing {
-            print("Error: One or more translations are missing.")
-            exit(1)
+            throw SwiftPolyglotError.missingTranslations
         } else if missingTranslations {
             print("Completed with missing translations.")
         } else {

Sources/SwiftPolyglotCore/SwiftPolyglotError.swift

@@ -0,0 +1,17 @@
+import Foundation
+
+enum SwiftPolyglotError: Error {
+    case missingTranslations
+    case noLanguageCodes
+}
+
+extension SwiftPolyglotError: LocalizedError {
+    public var errorDescription: String? {
+        switch self {
+        case .missingTranslations:
+            return "Error: One or more translations are missing."
+        case .noLanguageCodes:
+            return "Usage: script.swift <language codes> [--errorOnMissing]"
+        }
+    }
+}