Add warning for loading config.ini from an older or new version

applies on startup and on `Config` > `Load Config From...`
also changed wording for corrupted config.ini warning

src/BizHawk.Client.Common/config/ConfigService.cs

@@ -1,7 +1,11 @@
 using System;
 using System.IO;
 using System.Reflection;
+
+using BizHawk.Common;
+
 using Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
 using Newtonsoft.Json.Serialization;
 
 #pragma warning disable 618
@@ -30,6 +34,68 @@ namespace BizHawk.Client.Common
 			};
 		}
 
+		public static bool IsFromSameVersion(string filepath, out string msg)
+		{
+			// "2.5.1" => 0x02050100
+			static int VersionStrToInt(string s)
+			{
+				var a = s.Split('.');
+				var v = 0;
+				var i = 0;
+				while (i < 4)
+				{
+					v <<= 8;
+					if (i < a.Length) v += byte.TryParse(a[i], out var b) ? b : 0;
+					i++;
+				}
+				return v;
+			}
+			const string MSGFMT_NEWER = "Your config file ({0}) is from a newer version of EmuHawk, {2} (this is {1}). It may fail to load.";
+			const string MSGFMT_OLDER = "Your config file ({0}) is from an older version of EmuHawk, {2} (this is {1}). It may fail to load.";
+			const string MSGFMT_PRE_2_3_3 = "Your config file ({0}) is corrupted, or is from an older version of EmuHawk, predating 2.3.3 (this is {1}). It may fail to load.";
+			const string MSGFMT_PRE_2_5 = "Your config file ({0}) is corrupted, or is from an older version of EmuHawk, predating 2.5 (this is {1}). It may fail to load.";
+
+			if (!new FileInfo(filepath).Exists)
+			{
+				msg = null;
+				return true;
+			}
+			string cfgVersionStr = null;
+			try
+			{
+				cfgVersionStr = JObject.Parse(File.ReadAllText(filepath))["LastWrittenFrom"]?.Value<string>();
+			}
+			catch (Exception)
+			{
+				// ignored
+			}
+			if (cfgVersionStr == VersionInfo.MainVersion)
+			{
+				msg = null;
+				return true;
+			}
+			string fmt;
+			if (cfgVersionStr == null)
+			{
+				fmt = MSGFMT_PRE_2_3_3;
+			}
+			else
+			{
+				var cfgVersion = VersionStrToInt(cfgVersionStr);
+				if (cfgVersion < 0x02050000)
+				{
+					fmt = MSGFMT_PRE_2_5;
+				}
+				else
+				{
+					var thisVersion = VersionStrToInt(VersionInfo.MainVersion);
+					fmt = cfgVersion < thisVersion ? MSGFMT_OLDER : MSGFMT_NEWER;
+				}
+			}
+			msg = string.Format(fmt, Path.GetFileName(filepath), VersionInfo.MainVersion, cfgVersionStr);
+			return false;
+		}
+
 		/// <exception cref="InvalidOperationException">internal error</exception>
 		public static T Load<T>(string filepath) where T : new()
 		{

src/BizHawk.Client.EmuHawk/MainForm.cs

@@ -2783,6 +2783,10 @@ namespace BizHawk.Client.EmuHawk
 
 		private void LoadConfigFile(string iniPath)
 		{
+			if (!VersionInfo.DeveloperBuild && !ConfigService.IsFromSameVersion(iniPath, out var msg))
+			{
+				new MsgBox(msg, "Mismatched version in config file", MessageBoxIcon.Warning).ShowDialog();
+			}
 			Config = ConfigService.Load<Config>(iniPath);
 			Config.ResolveDefaults();
 			InitControls(); // rebind hotkeys

src/BizHawk.Client.EmuHawk/Program.cs

@@ -11,6 +11,7 @@ using Microsoft.VisualBasic.ApplicationServices;
 using BizHawk.Common;
 using BizHawk.Common.PathExtensions;
 using BizHawk.Client.Common;
+using BizHawk.Client.EmuHawk.CustomControls;
 
 using OSTC = EXE_PROJECT.OSTailoredCode;
 
@@ -110,12 +111,20 @@ namespace BizHawk.Client.EmuHawk
 
 			try
 			{
+				if (!VersionInfo.DeveloperBuild && !ConfigService.IsFromSameVersion(Config.DefaultIniPath, out var msg))
+				{
+					new MsgBox(msg, "Mismatched version in config file", MessageBoxIcon.Warning).ShowDialog();
+				}
 				GlobalWin.Config = ConfigService.Load<Config>(Config.DefaultIniPath);
 			}
 			catch (Exception e)
 			{
-				new ExceptionBox(e).ShowDialog();
-				new ExceptionBox("Since your config file is corrupted or from a different BizHawk version, we're going to recreate it. Back it up before proceeding if you want to investigate further.").ShowDialog();
+				new ExceptionBox(string.Join("\n",
+					"It appears your config file (config.ini) is corrupted; an exception was thrown while loading it.",
+					"On closing this warning, EmuHawk will delete your config file and generate a new one. You can go make a backup now if you'd like to look into diffs.",
+					"The caught exception was:",
+					e.ToString()
+				)).ShowDialog();
 				File.Delete(Config.DefaultIniPath);
 				GlobalWin.Config = ConfigService.Load<Config>(Config.DefaultIniPath);
 			}