Fix failure states in find_marker_on_or_before

Author: YoshiRulz

src/BizHawk.Client.EmuHawk/tools/Lua/Libraries/TAStudioLuaLibrary.cs

@@ -490,12 +490,12 @@ public void LoadBranch(int index)
 		[LuaMethod(
 			name: "find_marker_on_or_before",
 			description: "Returns the frame number of the marker closest to the given frame (including that frame, but not after it)."
-				+ " This may be the power-on marker at 0."
+				+ " This may be the power-on marker at 0. Returns -1 if the arguments are invalid or TAStudio isn't active."
 				+ " If branchID is specified, searches the markers in that branch instead.")]
 		public int FindMarkerOnOrBefore(int frame, string/*?*/ branchID = null)
 			=> Engaged() && MarkerListForBranch(branchID) is TasMovieMarkerList markers
-				? markers.PreviousOrCurrent(frame).Frame
-				: default;
+				? markers.PreviousOrCurrent(frame)?.Frame ?? -1
+				: -1;
 
 		[LuaMethodExample("""
 			local marker_label = tastudio.getmarker(tastudio.get_frames_with_markers()[2]);