Verify the entered CK3 mods path after reading the configuration (#2251)

Sentry event ID: 92f7299db51a4442bbf9b1a00783fc5c

ImperatorToCK3/Configuration.cs

@@ -48,6 +48,7 @@ public Configuration(ConverterVersion converterVersion) {
 		VerifyCK3Path();
 		VerifyCK3Version(converterVersion);
 		VerifyImperatorDocPath();
+		VerifyCK3ModsPath();
 
 		Logger.IncrementProgress();
 	}
@@ -236,6 +237,21 @@ private void VerifyImperatorDocPath() {
 
 		Logger.Debug($"I:R documents path {ImperatorPath} is valid.");
 	}
+
+	private void VerifyCK3ModsPath() {
+		if (!Directory.Exists(CK3ModsPath)) {
+			throw new UserErrorException($"{CK3ModsPath} does not exist!");
+		}
+
+		// If the mods folder contains any files, at least one on them should have a .mod extension.
+		var filesInFolder = Directory.GetFiles(CK3ModsPath);
+		if (filesInFolder.Length > 0) {
+			var modFiles = filesInFolder.Where(f => f.EndsWith(".mod", StringComparison.OrdinalIgnoreCase));
+			if (!modFiles.Any()) {
+				throw new UserErrorException($"{CK3ModsPath} does not contain any .mod files!");
+			}
+		}
+	}
 
 	private void SetOutputName() {
 		if (string.IsNullOrWhiteSpace(OutputModName)) {