Turn up various Error Prone checks (#710)

I ran into violations of some of these issues when collaborating on another change. I think they are all fairly uncontroversial.

build.gradle

@@ -71,10 +71,16 @@ subprojects { project ->
                 check("StringSplitter", CheckSeverity.OFF)
                 // https://github.com/google/error-prone/issues/3366
                 check("CanIgnoreReturnValueSuggester", CheckSeverity.OFF)
+                // turn up various checks
                 check("WildcardImport", CheckSeverity.ERROR)
                 check("MissingBraces", CheckSeverity.ERROR)
                 check("TypeToString", CheckSeverity.ERROR)
                 check("SymbolToString", CheckSeverity.ERROR)
+                check("MultipleTopLevelClasses", CheckSeverity.ERROR)
+                check("ClassName", CheckSeverity.ERROR)
+                check("PackageLocation", CheckSeverity.ERROR)
+                check("UnnecessaryAnonymousClass", CheckSeverity.ERROR)
+                check("UnusedException", CheckSeverity.ERROR)
             }
         } else {
             // Disable Error Prone checks on JDK 8, as more recent Error Prone versions don't run on JDK 8

nullaway/src/test/java/com/uber/nullaway/tools/SerializationTestHelper.java

@@ -90,8 +90,8 @@ public void doTest() {
     Path outputPath = outputDir.resolve(fileName);
     try {
       Files.deleteIfExists(outputPath);
-    } catch (IOException ignored) {
-      throw new RuntimeException("Failed to delete older file at: " + outputPath);
+    } catch (IOException e) {
+      throw new RuntimeException("Failed to delete older file at: " + outputPath, e);
     }
     compilationTestHelper.doTest();
     List<T> actualOutputs = readActualOutputs(outputPath);