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/BacktraceCurrent.cpp b/libbacktrace/BacktraceCurrent.cpp
index fb76b85..474d099 100644
--- a/libbacktrace/BacktraceCurrent.cpp
+++ b/libbacktrace/BacktraceCurrent.cpp
@@ -67,11 +67,11 @@
 bool BacktraceCurrent::Unwind(size_t num_ignore_frames, ucontext_t* ucontext) {
   if (GetMap() == nullptr) {
     // Without a map object, we can't do anything.
-    error_ = BACKTRACE_UNWIND_ERROR_MAP_MISSING;
+    error_.error_code = BACKTRACE_UNWIND_ERROR_MAP_MISSING;
     return false;
   }
 
-  error_ = BACKTRACE_UNWIND_NO_ERROR;
+  error_.error_code = BACKTRACE_UNWIND_NO_ERROR;
   if (ucontext) {
     return UnwindFromContext(num_ignore_frames, ucontext);
   }
@@ -163,7 +163,7 @@
     BACK_ASYNC_SAFE_LOGE("sigaction failed: %s", strerror(errno));
     ThreadEntry::Remove(entry);
     pthread_mutex_unlock(&g_sigaction_mutex);
-    error_ = BACKTRACE_UNWIND_ERROR_INTERNAL;
+    error_.error_code = BACKTRACE_UNWIND_ERROR_INTERNAL;
     return false;
   }
 
@@ -171,9 +171,9 @@
     // Do not emit an error message, this might be expected. Set the
     // error and let the caller decide.
     if (errno == ESRCH) {
-      error_ = BACKTRACE_UNWIND_ERROR_THREAD_DOESNT_EXIST;
+      error_.error_code = BACKTRACE_UNWIND_ERROR_THREAD_DOESNT_EXIST;
     } else {
-      error_ = BACKTRACE_UNWIND_ERROR_INTERNAL;
+      error_.error_code = BACKTRACE_UNWIND_ERROR_INTERNAL;
     }
 
     sigaction(THREAD_SIGNAL, &oldact, nullptr);
@@ -218,9 +218,9 @@
   } else {
     // Check to see if the thread has disappeared.
     if (tgkill(Pid(), Tid(), 0) == -1 && errno == ESRCH) {
-      error_ = BACKTRACE_UNWIND_ERROR_THREAD_DOESNT_EXIST;
+      error_.error_code = BACKTRACE_UNWIND_ERROR_THREAD_DOESNT_EXIST;
     } else {
-      error_ = BACKTRACE_UNWIND_ERROR_THREAD_TIMEOUT;
+      error_.error_code = BACKTRACE_UNWIND_ERROR_THREAD_TIMEOUT;
       BACK_ASYNC_SAFE_LOGE("Timed out waiting for signal handler to get ucontext data.");
     }
   }