Add error description in KeychainError (#21)

Sources/Keychain/KeychainError.swift

@@ -1,10 +1,20 @@
 #if canImport(Security)
 
 import Security
+import Foundation
 
-enum KeychainError: Error {
+enum KeychainError: LocalizedError {
   case keychain(OSStatus)
   case invalidResult
+
+  var errorDescription: String? {
+    switch self {
+    case .keychain(let oSStatus):
+      return "Keychain Error: OSStatus=\(oSStatus)."
+    case .invalidResult:
+      return "Keychain Error: Invalid result."
+    }
+  }
 }
 
 #endif

Tests/Keychain/KeychainErrorTests.swift

@@ -0,0 +1,22 @@
+#if canImport(Security)
+
+@testable import KeychainStore
+
+import Foundation
+import XCTest
+
+final class KeychainErrorTests: XCTestCase {
+  func testErrorDescription() {
+    XCTAssertEqual(
+      KeychainError.invalidResult.errorDescription,
+      "Keychain Error: Invalid result."
+    )
+
+    XCTAssertEqual(
+      KeychainError.keychain(0).errorDescription,
+      "Keychain Error: OSStatus=0."
+    )
+  }
+}
+
+#endif