Add LocalizedError conformance to ContainerizationError (#528)

Without `LocalizedError` conformance, calling `localizedDescription` on
a `ContainerizationError` falls back to the default representation
(e.g., "ContainerizationError.ContainerizationError error 1.") instead
of returning the actual error message. This makes it difficult to show
meaningful errors in a UI, where `localizedDescription` is the standard
way to get a user-facing string.

Sources/ContainerizationError/ContainerizationError.swift

@@ -14,11 +14,13 @@
 // limitations under the License.
 //===----------------------------------------------------------------------===//
 
+import Foundation
+
 /// The core error type for Containerization.
 ///
 /// Most API surfaces for the core container/process/agent types will
 /// return a ContainerizationError.
-public struct ContainerizationError: Swift.Error, Sendable {
+public struct ContainerizationError: Swift.Error, LocalizedError, Sendable {
     /// A code describing the error encountered.
     public var code: Code
     /// A description of the error.
@@ -65,6 +67,10 @@ public struct ContainerizationError: Swift.Error, Sendable {
     public func isCode(_ code: Code) -> Bool {
         self.code == code
     }
+
+    public var errorDescription: String? {
+        message
+    }
 }
 
 extension ContainerizationError: CustomStringConvertible {