Improve UX when there are no optional mods

src/main/kotlin/link/infra/packwiz/installer/UpdateManager.kt

@@ -255,8 +255,7 @@ class UpdateManager internal constructor(private val opts: Options, val ui: IUse
 			return
 		}
 		if (!indexFileSource.hashIsEqual(indexHash)) {
-			// TODO: throw exception
-			println("I was meant to put an error message here but I'll do that later")
+			ui.handleExceptionAndExit(RuntimeException("Your index hash is invalid! Please run packwiz refresh on the pack again"))
 			return
 		}
 		if (stateHandler.cancelButton) {

src/main/kotlin/link/infra/packwiz/installer/request/handlers/RequestHandlerHTTP.kt

@@ -4,6 +4,7 @@ import link.infra.packwiz.installer.metadata.SpaceSafeURI
 import link.infra.packwiz.installer.request.IRequestHandler
 import okio.Source
 import okio.source
+import java.net.HttpURLConnection
 
 open class RequestHandlerHTTP : IRequestHandler {
 	override fun matchesHandler(loc: SpaceSafeURI): Boolean {
@@ -12,14 +13,17 @@ open class RequestHandlerHTTP : IRequestHandler {
 	}
 
 	override fun getFileSource(loc: SpaceSafeURI): Source? {
-		val conn = loc.toURL().openConnection()
+		val conn = loc.toURL().openConnection() as HttpURLConnection
 		// TODO: when do we send specific headers??? should there be a way to signal this?
-		// github *sometimes* requires it, sometimes not!
-		//conn.addRequestProperty("Accept", "application/octet-stream");
+		conn.addRequestProperty("Accept", "application/octet-stream")
+		// TODO: include version?
+		conn.addRequestProperty("User-Agent", "packwiz-installer")
+
 		conn.apply {
 			// 30 second read timeout
 			readTimeout = 30 * 1000
+			requestMethod = "GET"
 		}
-		return conn.getInputStream().source()
+		return conn.inputStream.source()
 	}
 }

src/main/kotlin/link/infra/packwiz/installer/ui/InstallWindow.kt

@@ -180,9 +180,16 @@ class InstallWindow : IUserInterface {
 	override fun showOptions(options: List<IOptionDetails>): Future<Boolean> {
 		val future = CompletableFuture<Boolean>()
 		EventQueue.invokeLater {
-			OptionsSelectWindow(options, future, frmPackwizlauncher).apply {
-				defaultCloseOperation = JDialog.DISPOSE_ON_CLOSE
-				isVisible = true
+			if (options.isEmpty()) {
+				JOptionPane.showMessageDialog(null,
+					"This modpack has no optional mods!",
+					"Optional mods", JOptionPane.INFORMATION_MESSAGE)
+				future.complete(false)
+			} else {
+				OptionsSelectWindow(options, future, frmPackwizlauncher).apply {
+					defaultCloseOperation = JDialog.DISPOSE_ON_CLOSE
+					isVisible = true
+				}
 			}
 		}
 		return future
@@ -201,7 +208,7 @@ class InstallWindow : IUserInterface {
 
 	override fun disableOptionsButton() {
 		btnOptions.apply {
-			text = "Optional mods..."
+			text = "No optional mods"
 			isEnabled = false
 		}
 	}