Make the error message's corner rounded

to match the style of other messaging elements in the page

Bug: 206859554
Test: locally
Change-Id: I39389f93fd9123a0cd8370ab14878b41917687c1
diff --git a/host/frontend/webrtc/client/style.css b/host/frontend/webrtc/client/style.css
index ce673af..8fdd054 100644
--- a/host/frontend/webrtc/client/style.css
+++ b/host/frontend/webrtc/client/style.css
@@ -79,6 +79,7 @@
   font-family: 'Open Sans', sans-serif;
   padding: 10px;
   margin: 10px;
+  border-radius: 10px;
 }
 #error-message.hidden {
   display: none;