Get rid of custom ExceptionRule methods

The only remaining methods on ExceptionRule after this are methods that
also exist on ExpectedException, which will allow us to, in the next CL,
swap out the one for the other and then run the automated refactoring to
turn it all into assertThrows/expectThrows.

Note that there were some assertions about root causes that couldn't
easily be turned into ExpectedException invocations, so I simply
converted them directly to usages of assertThrows/expectThrows.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=178623431

javatests/google/registry/util/RetrierTest.java

@@ -86,13 +86,15 @@ public class RetrierTest {
 
   @Test
   public void testRetryableException() throws Exception {
-    thrown.expect(CountingException.class, "3");
+    thrown.expect(CountingException.class);
+    thrown.expectMessage("3");
     retrier.callWithRetry(new CountingThrower(3), CountingException.class);
   }
 
   @Test
   public void testUnretryableException() throws Exception {
-    thrown.expect(CountingException.class, "1");
+    thrown.expect(CountingException.class);
+    thrown.expectMessage("1");
     retrier.callWithRetry(new CountingThrower(5), IllegalArgumentException.class);
   }
 
@@ -104,7 +106,8 @@ public class RetrierTest {
 
   @Test
   public void testRetryFailed_withReporter() throws Exception {
-    thrown.expect(CountingException.class, "3");
+    thrown.expect(CountingException.class);
+    thrown.expectMessage("3");
     TestReporter reporter = new TestReporter();
     try {
       retrier.callWithRetry(new CountingThrower(3), reporter, CountingException.class);