feat: handle errors better

Author: MiniDigger

src/main/java/io/papermc/patchroulette/controller/RESTController.java

@@ -5,9 +5,11 @@
 import io.papermc.patchroulette.service.PatchService;
 import java.util.List;
 import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
 import org.springframework.security.access.prepost.PreAuthorize;
 import org.springframework.security.core.Authentication;
+import org.springframework.web.bind.annotation.ExceptionHandler;
 import org.springframework.web.bind.annotation.GetMapping;
 import org.springframework.web.bind.annotation.PostMapping;
 import org.springframework.web.bind.annotation.RequestBody;
@@ -126,4 +128,9 @@ public ResponseEntity<String> undoPatch(final Authentication auth, @RequestBody
         this.patchService.undoPatch(input, user);
         return ResponseEntity.ok("Patch moved to WIP.");
     }
+
+    @ExceptionHandler(IllegalStateException.class)
+    public ResponseEntity<String> handleException(final IllegalStateException e) {
+        return ResponseEntity.status(HttpStatus.CONFLICT).body(e.getMessage());
+    }
 }