Fix posting exceptions when a debugger is attached, fix UpdateDebugger, fully implement ThreadGroupReference.Children.

Posting exceptions and UpdateDebugger were broken by the recent stack-walking
rewrite.

dalvikvm never did ThreadGroupReference.Children correctly; it only admitted
that "system" is the parent of "main".

Change-Id: I386f2fa5e01fba56cb2a1af4f136dbf31da0ff07
diff --git a/src/monitor.cc b/src/monitor.cc
index e5e867a..a7d08bc 100644
--- a/src/monitor.cc
+++ b/src/monitor.cc
@@ -910,7 +910,7 @@
 
   // Is there any reason to believe there's any synchronization in this method?
   const DexFile::CodeItem* code_item = mh.GetCodeItem();
-  CHECK(code_item != NULL);
+  CHECK(code_item != NULL) << PrettyMethod(m);
   if (code_item->tries_size_ == 0) {
     return; // No "tries" implies no synchronization, so no held locks to report.
   }