Better error reporting, fix NPE (stupid Gson)

src/main/java/link/infra/packwiz/installer/Main.java

@@ -122,7 +122,13 @@ public class Main {
 			ui.executeManager(new Runnable(){
 				@Override
 				public void run() {
-					new UpdateManager(uOptions, ui);
+					try {
+						new UpdateManager(uOptions, ui);
+					} catch (Exception e) {
+						// TODO: better error message?
+						ui.handleExceptionAndExit(e);
+						return;
+					}
 				}
 			});
 		} catch (Exception e) {

src/main/java/link/infra/packwiz/installer/UpdateManager.java

@@ -110,16 +110,18 @@ public class UpdateManager {
 
 		ui.submitProgress(new InstallProgress("Loading manifest file..."));
 		Gson gson = new Gson();
-		ManifestFile manifest;
+		ManifestFile manifest = null;
 		try {
 			manifest = gson.fromJson(new FileReader(Paths.get(opts.packFolder, opts.manifestFile).toString()),
 					ManifestFile.class);
-		} catch (FileNotFoundException e) {
+		} catch (FileNotFoundException e) { // Do nothing
-			manifest = new ManifestFile();
 		} catch (JsonSyntaxException | JsonIOException e) {
 			ui.handleExceptionAndExit(e);
 			return;
 		}
+		if (manifest == null) {
+			manifest = new ManifestFile();
+		}
 
 		ui.submitProgress(new InstallProgress("Loading pack file..."));
 		GeneralHashingSource packFileSource;