Insert failure check after processing missing definitions.

Change-Id: Ia19df7837b3a3902bc70ae48f05908b6d2291499

src/main/java/com/android/tools/r8/shaking/MissingClasses.java

diff --git a/src/main/java/com/android/tools/r8/shaking/MissingClasses.java b/src/main/java/com/android/tools/r8/shaking/MissingClasses.java
index 7d23fa4..2fa2e2c 100644
--- a/src/main/java/com/android/tools/r8/shaking/MissingClasses.java
+++ b/src/main/java/com/android/tools/r8/shaking/MissingClasses.java
@@ -142,6 +142,7 @@
           appView.reporter().error(diagnostic);
         }
       }
+      appView.reporter().failIfPendingErrors();
       return build();
     }