Reworked this change a bit.

- GHApiInfo need not be public because it's not publicly exposed.
- Throwing an exception is better IMO as it allows richer error message,
  including the differentiation between unreachable host name vs wrong
  URL, and reporting the API endpoint URL that was actually tried.

src/test/java/org/kohsuke/github/GitHubTest.java

@@ -124,6 +124,6 @@ public class GitHubTest {
     @Test
     public void testGitHubIsApiUrlValid() throws IOException {
         GitHub github = GitHub.connectAnonymously();
-        assertTrue(github.isApiUrlValid());
+        github.checkApiUrlValidity();
     }
 }