添加对 Core 的直接引用

RhythmBaseCore/Exceptions/InvalidRDBeatException.cs

@@ -0,0 +1,21 @@
+namespace RhythmBase.Exceptions
+{
+	/// <summary>
+	/// Exception thrown when an invalid RD beat is encountered.
+	/// </summary>
+	public class InvalidRDBeatException : RhythmBaseException
+	{
+		/// <summary>
+		/// Initializes a new instance of the <see cref="InvalidRDBeatException"/> class.
+		/// </summary>
+		public InvalidRDBeatException()
+		{
+			Message = "The beat is invalid, possibly because the beat is not associated with the RDLevel.";
+		}
+
+		/// <summary>
+		/// Gets the error message that explains the reason for the exception.
+		/// </summary>
+		public override string Message { get; }
+	}
+}