Refactor EmuClientApi.ShowFuture, throwing on invalid arg

fixes 6223c9d

Author: YoshiRulz

src/BizHawk.Client.Common/Api/Classes/EmuClientApi.cs

@@ -221,14 +221,16 @@ public void SetWindowSize(int size)
 
 		public void ShowFuture(ShowFutureCallback/*?*/ preFrameCallback, short maxFrames)
 		{
-			if (maxFrames < 1 && preFrameCallback != null)
+			if (preFrameCallback is not null)
 			{
-				_logCallback($"Invalid number of future frames ({maxFrames}); number must be positive.");
-				return;
+				if (maxFrames <= 0) throw new ArgumentOutOfRangeException(paramName: nameof(maxFrames), maxFrames, message: "Invalid number of future frames, number must be positive.");
+				_mainForm.MaxFutureFrames = maxFrames;
+			}
+			else
+			{
+				_mainForm.MaxFutureFrames = 0;
 			}
-
 			_mainForm.PreFutureFrameCallback = preFrameCallback;
-			_mainForm.MaxFutureFrames = maxFrames;
 		}
 
 		public void SpeedMode(int percent)

src/BizHawk.Client.Common/Api/Interfaces/IEmuClientApi.cs

@@ -145,6 +145,10 @@ public interface IEmuClientApi : IDisposable, IExternalApi
 		/// When the callback returns true, emulation will rewind to the real current frame and the just-run future frame will be displayed.
 		/// <br/>Pass null to disable future frame display.</param>
 		/// <param name="maxFrames">The maximum number of future frames to emulate. Useful to avoid freezing the client UI in case of accidentally never returning true from the callback.</param>
+		/// <exception cref="ArgumentOutOfRangeException">
+		/// <paramref name="maxFrames"/> ≤ <c>0</c>
+		/// (and <paramref name="preFrameCallback"/> is a delegate, since <paramref name="maxFrames"/> is ignored when it's <see langword="null"/>)
+		/// </exception>
 		void ShowFuture(ShowFutureCallback/*?*/ preFrameCallback, short maxFrames);
 
 		void SpeedMode(int percent);