Make errorprone warnings an error so they are caught during tests.

Bug:
Change-Id: Ib8bffe09ad04cdb6af15f0b1aaec0fd4f2594535
diff --git a/build.gradle b/build.gradle
index d93cbf1..c351aca 100644
--- a/build.gradle
+++ b/build.gradle
@@ -16,23 +16,23 @@
     '-XepDisableAllChecks',
     // D8 want to use reference equality, thus disable the checker explicitly
     '-Xep:ReferenceEquality:OFF',
-    '-Xep:ClassCanBeStatic:WARN',
-    '-Xep:OperatorPrecedence:WARN',
-    '-Xep:RemoveUnusedImports:WARN',
-    '-Xep:MissingOverride:WARN',
-    '-Xep:OvershadowingSubclassFields:WARN',
-    '-Xep:IntLongMath:WARN',
-    '-Xep:EqualsHashCode:WARN',
-    '-Xep:InconsistentOverloads:WARN',
-    '-Xep:ArrayHashCode:WARN',
-    '-Xep:EqualsIncompatibleType:WARN',
-    '-Xep:NonOverridingEquals:WARN',
-    '-Xep:FallThrough:WARN',
-    '-Xep:MissingCasesInEnumSwitch:WARN',
-    '-Xep:MissingDefault:WARN',
-    '-Xep:MultipleTopLevelClasses:WARN',
-    '-Xep:NarrowingCompoundAssignment:WARN',
-    '-Xep:BoxedPrimitiveConstructor:WARN']
+    '-Xep:ClassCanBeStatic:ERROR',
+    '-Xep:OperatorPrecedence:ERROR',
+    '-Xep:RemoveUnusedImports:ERROR',
+    '-Xep:MissingOverride:ERROR',
+    '-Xep:OvershadowingSubclassFields:ERROR',
+    '-Xep:IntLongMath:ERROR',
+    '-Xep:EqualsHashCode:ERROR',
+    '-Xep:InconsistentOverloads:ERROR',
+    '-Xep:ArrayHashCode:ERROR',
+    '-Xep:EqualsIncompatibleType:ERROR',
+    '-Xep:NonOverridingEquals:ERROR',
+    '-Xep:FallThrough:ERROR',
+    '-Xep:MissingCasesInEnumSwitch:ERROR',
+    '-Xep:MissingDefault:ERROR',
+    '-Xep:MultipleTopLevelClasses:ERROR',
+    '-Xep:NarrowingCompoundAssignment:ERROR',
+    '-Xep:BoxedPrimitiveConstructor:ERROR']
 
 apply from: 'copyAdditionalJctfCommonFiles.gradle'