Exit 2 instead of 1 when the tool's arguments are used incorrectly

core/src/main/java/com/google/googlejavaformat/java/Main.java

@@ -79,7 +79,8 @@ static int main(PrintWriter out, PrintWriter err, String... args) {
       return formatter.format(args);
     } catch (UsageException e) {
       err.print(e.getMessage());
-      return 1;
+      // We return exit code 2 to differentiate usage issues from code formatting issues.
+      return 2;
     } finally {
       err.flush();
       out.flush();