Push ReceiveError advice into structured exception type

The ReceiveCommits.ReceiveError is very specific to the
DefaultPermissionBackend. Provide a way for other backend
implementations to provide their own advice, by enriching the
AuthException hierarchy.

We now have a PermissionDeniedException which records up to three items:
 * The description of the permission, as returned by
   GerritPermission#describeForException.
 * Free-form advice for addressing the issue.
 * The resource name, if applicable.

Now, ReceiveCommits doesn't have to know about the ReceiveError text;
DefaultPermissionBackend can just set the advice on any AuthExceptions
it throws. Currently, the advice is only displayed over the wire during
ReceiveCommits, but one could imagine that it might be helpful in bare
HTTP responses as well, for example.

Update the push permissions test to account for the new messages. Note
that they are much more consistent and follow a common format, which is
not surprising since a goal of this change was to eliminate much of the
hard-coding in ReceiveCommits.

There are a few tests in PushPermisisonsIT which still show hard-coded
error messages. These will require a bit more work to make sure they get
understandable messages and advice.

Change-Id: I34dc173ffd040829d8ff2853d4a728fa323137b5

java/com/google/gerrit/git/testing/PushResultSubject.java

@@ -54,6 +54,13 @@ public class PushResultSubject extends Subject<PushResultSubject, PushResult> {
     Truth.assertThat(trimMessages()).isEqualTo(Arrays.stream(expectedLines).collect(joining("\n")));
   }
 
+  public void containsMessages(String... expectedLines) {
+    checkArgument(expectedLines.length > 0, "use hasNoMessages()");
+    isNotNull();
+    Iterable<String> got = Splitter.on("\n").split(trimMessages());
+    Truth.assertThat(got).containsAllIn(expectedLines).inOrder();
+  }
+
   private String trimMessages() {
     return trimMessages(actual().getMessages());
   }