add error description

Signed-off-by: L1MeN9Yu <baal998.mophisto@gmail.com>

Elva.Mac.CoimmandLine.Demo/main.swift

@@ -9,4 +9,4 @@
 import Foundation
 import Elva
 
-Elva.setup(environment: Env.self)
+Elva.register(environment: Env.self)

Source/Frontend/Elva.swift

@@ -9,7 +9,7 @@
 
 private(set) var __environment: Environment.Type?
 
-public func setup(environment: Environment.Type) {
+public func register(environment: Environment.Type) {
     __environment = environment
     elva_setup { flag, file, function, line, message in
         guard let logFlag = LogFlag(unsignedIntValue: CUnsignedInt(flag)),

Source/Frontend/Error.swift

@@ -9,3 +9,16 @@ public enum Error: Swift.Error {
     case compress
     case decompress
 }
+
+extension Error: CustomStringConvertible {
+    public var description: String {
+        switch self {
+        case .compress:
+            return "compress error"
+        case .decompress:
+            return "decompress error"
+        @unknown default:
+            return "unknown error"
+        }
+    }
+}