libbacktrace: export offline unwinding failures.

This is to help debugging different offline unwiding failures.

Bug: http://b/69383534
Test: run backtrace_test.
Change-Id: I5ed4837027a9f17d032925e97e9f5927161444b3
diff --git a/libbacktrace/UnwindCurrent.cpp b/libbacktrace/UnwindCurrent.cpp
index 2c87fa8..3ccf13c 100644
--- a/libbacktrace/UnwindCurrent.cpp
+++ b/libbacktrace/UnwindCurrent.cpp
@@ -81,7 +81,7 @@
     int ret = unw_getcontext(&context_);
     if (ret < 0) {
       BACK_LOGW("unw_getcontext failed %d", ret);
-      error_ = BACKTRACE_UNWIND_ERROR_SETUP_FAILED;
+      error_.error_code = BACKTRACE_UNWIND_ERROR_SETUP_FAILED;
       return false;
     }
   } else {
@@ -93,7 +93,7 @@
   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;
+    error_.error_code = BACKTRACE_UNWIND_ERROR_SETUP_FAILED;
     return false;
   }
   initialized_ = true;