Add error reporting mechanism for failing Unwind.

Remove the logging of an error if a thread disappears before the unwind
can begin. This can happen, so allow the caller to determine if this
is really a problem worth logging.

Bug: 27449879

(cherry picked from commit 206a3b9798e3622c906a3cafdb113c271c1c927c)

Change-Id: If9e7cfeb6eb7b122679a734c1a9eacee8354ef18
diff --git a/libbacktrace/UnwindCurrent.cpp b/libbacktrace/UnwindCurrent.cpp
index 67e583f..666c481 100644
--- a/libbacktrace/UnwindCurrent.cpp
+++ b/libbacktrace/UnwindCurrent.cpp
@@ -70,6 +70,7 @@
     int ret = unw_getcontext(&context_);
     if (ret < 0) {
       BACK_LOGW("unw_getcontext failed %d", ret);
+      error_ = BACKTRACE_UNWIND_ERROR_SETUP_FAILED;
       return false;
     }
   } else {
@@ -81,6 +82,7 @@
   int ret = unw_init_local(cursor.get(), &context_);
   if (ret < 0) {
     BACK_LOGW("unw_init_local failed %d", ret);
+    error_ = BACKTRACE_UNWIND_ERROR_SETUP_FAILED;
     return false;
   }