Add ParsableArguments#_errorPrefix to allow a more-customized error prefix than ParsableArguments#_errorLabel. (#837)

Author: rgoldberg

Sources/ArgumentParser/Parsable Types/ParsableArguments.swift

@@ -25,7 +25,15 @@ public protocol ParsableArguments: Decodable, _SendableMetatype {
   mutating func validate() throws
 
   /// The label to use for "Error: ..." messages from this type (experimental).
+  ///
+  /// Can be ignored if `_errorPrefix`'s is changed.
+  @available(*, deprecated, message: "Use _errorPrefix instead.")
   static var _errorLabel: String { get }
+
+  /// The prefix to use for "Error: ..." messages from this type (experimental).
+  ///
+  /// Defaults to `"\(_errorLabel): "`.
+  static var _errorPrefix: String { get }
 }
 
 /// A type that provides the `ParsableCommand` interface to a `ParsableArguments` type.
@@ -60,6 +68,13 @@ extension ParsableArguments {
   public static var _errorLabel: String {
     "Error"
   }
+
+  /// The prefix to use for "Error: ..." messages from this type (experimental).
+  ///
+  /// Defaults to `"\(_errorLabel): "`.
+  public static var _errorPrefix: String {
+    "\(_errorLabel): "
+  }
 }
 
 // MARK: - API

Sources/ArgumentParser/Usage/MessageInfo.swift

@@ -164,10 +164,10 @@ enum MessageInfo {
     case .validation(let message, let usage, let help):
       let helpMessage = help.isEmpty ? "" : "Help:  \(help)\n"
       let errorMessage =
-        message.isEmpty ? "" : "\(args._errorLabel): \(message)\n"
+        message.isEmpty ? "" : "\(args._errorPrefix)\(message)\n"
       return errorMessage + helpMessage + usage
     case .other(let message, _):
-      return message.isEmpty ? "" : "\(args._errorLabel): \(message)"
+      return message.isEmpty ? "" : "\(args._errorPrefix)\(message)"
     }
   }