ErrorProne: Enable and fix UnusedException check

Change-Id: I9fd9dac191f1bcaf2653fa9adfc7dac3cf5715ce

tools/BUILD

diff --git a/tools/BUILD b/tools/BUILD
index ab2b9b3..5cfe48c 100644
--- a/tools/BUILD
+++ b/tools/BUILD
@@ -82,6 +82,7 @@
         "-Xep:TypeParameterUnusedInFormals:ERROR",
         "-Xep:URLEqualsHashCode:ERROR",
         "-Xep:UnsynchronizedOverridesSynchronized:ERROR",
+        "-Xep:UnusedException:ERROR",
         "-Xep:WaitNotInLoop:ERROR",
         "-Xep:WildcardImport:ERROR",
     ],