Add an experimental customization point for the error label (#223)

SwiftPM uses "error:" instead of "Error:" to make its error
messages match the compiler.

Author: natecook1000

Sources/ArgumentParser/Parsable Types/CommandConfiguration.swift

@@ -110,5 +110,3 @@ public struct CommandConfiguration {
     self.helpNames = helpNames
   }
 }
-
-

Sources/ArgumentParser/Parsable Types/ParsableArguments.swift

@@ -34,6 +34,9 @@ public protocol ParsableArguments: Decodable {
   /// Implement this method to perform validation or other processing after
   /// creating a new instance from command-line arguments.
   mutating func validate() throws
+  
+  /// The label to use for "Error: ..." messages from this type. (experimental)
+  static var _errorLabel: String { get }
 }
 
 /// A type that provides the `ParsableCommand` interface to a `ParsableArguments` type.
@@ -71,6 +74,10 @@ extension ParsableArguments {
   internal static var asCommand: ParsableCommand.Type {
     self as? ParsableCommand.Type ?? _WrappedParsableCommand<Self>.self
   }
+  
+  public static var _errorLabel: String {
+    "Error"
+  }
 }
 
 // MARK: - API
@@ -121,7 +128,7 @@ extension ParsableArguments {
   public static func fullMessage(
     for error: Error
   ) -> String {
-    MessageInfo(error: error, type: self).fullText
+    MessageInfo(error: error, type: self).fullText(for: self)
   }
 
   /// Returns the text of the help screen for this type.
@@ -174,11 +181,12 @@ extension ParsableArguments {
     }
 
     let messageInfo = MessageInfo(error: error, type: self)
-    if !messageInfo.fullText.isEmpty {
+    let fullText = messageInfo.fullText(for: self)
+    if !fullText.isEmpty {
       if messageInfo.shouldExitCleanly {
-        print(messageInfo.fullText)
+        print(fullText)
       } else {
-        print(messageInfo.fullText, to: &standardError)
+        print(fullText, to: &standardError)
       }
     }
     _exit(messageInfo.exitCode.rawValue)

Sources/ArgumentParser/Usage/MessageInfo.swift

@@ -120,15 +120,15 @@ enum MessageInfo {
     }
   }
 
-  var fullText: String {
+  func fullText(for args: ParsableArguments.Type) -> String {
     switch self {
     case .help(text: let text):
       return text
     case .validation(message: let message, usage: let usage):
-      let errorMessage = message.isEmpty ? "" : "Error: \(message)\n"
+      let errorMessage = message.isEmpty ? "" : "\(args._errorLabel): \(message)\n"
       return errorMessage + usage
     case .other(let message, _):
-      return message.isEmpty ? "" : "Error: \(message)"
+      return message.isEmpty ? "" : "\(args._errorLabel): \(message)"
     }
   }