core: silence libunwind errors

Every libunwind error during backtrace collection is reported with
`say_error`. Since commit 19abfd2a ("misc: get rid of fiber_gc")
backtraces are collected on each fiber gc allocation, of which there are
plenty.

For some reason (https://github.com/tarantool/tarantool/issues/7980)
each unw_step fails on mac, and an error is spammed to instance logs,
even though the backtrace is actually collected.

Silence the errors, since there is no much use for them anyway. And
silence all of them just to be consistent.

This doesn't close #7980, because that issue still needs a proper fix.
Although its severity is ameliorated now.

In-scope-of #7980

NO_DOC=bugfix
NO_CHANGELOG=bugfix
NO_TEST=nothing to test

(cherry picked from commit c324eedd)

src/lib/core/backtrace.c

@@ -65,13 +65,13 @@ collect_current_stack(struct backtrace *bt, void *stack)
 	unw_context_t unw_ctx;
 	int rc = unw_getcontext(&unw_ctx);
 	if (rc != 0) {
-		say_error("unwinding error: unw_getcontext failed");
+		say_debug("unwinding error: unw_getcontext failed");
 		return stack;
 	}
 	unw_cursor_t unw_cur;
 	rc = unw_init_local(&unw_cur, &unw_ctx);
 	if (rc != 0) {
-		say_error("unwinding error: unw_init_local failed");
+		say_debug("unwinding error: unw_init_local failed");
 		return stack;
 	}
 	for (unsigned frame_no = 0; frame_no < BACKTRACE_FRAME_COUNT_MAX;
@@ -79,7 +79,7 @@ collect_current_stack(struct backtrace *bt, void *stack)
 		unw_word_t ip;
 		rc = unw_get_reg(&unw_cur, UNW_REG_IP, &ip);
 		if (rc != 0) {
-			say_error("unwinding error: unw_get_reg failed with "
+			say_debug("unwinding error: unw_get_reg failed with "
 				  "status: %d", rc);
 			return stack;
 		}
@@ -87,7 +87,7 @@ collect_current_stack(struct backtrace *bt, void *stack)
 		rc = unw_step(&unw_cur);
 		if (rc <= 0) {
 			if (rc < 0)
-				say_error("unwinding error: unw_step failed "
+				say_debug("unwinding error: unw_step failed "
 					  "with status: %d", rc);
 			break;
 		}
@@ -260,14 +260,14 @@ backtrace_frame_resolve(const struct backtrace_frame *frame,
 				    proc_name_buf, TT_STATIC_BUF_LEN, offset,
 				    NULL);
 	if (rc != 0) {
-		say_error("unwinding error: `get_proc_name` accessor failed: "
+		say_debug("unwinding error: `get_proc_name` accessor failed: "
 			  "%s", unw_strerror(rc));
 		return NULL;
 	}
 #else /* __APPLE__ */
 	Dl_info dli;
 	if (dladdr(frame->ip, &dli) == 0) {
-		say_error("unwinding error: `dladdr` failed");
+		say_debug("unwinding error: `dladdr` failed");
 		return NULL;
 	}
 
@@ -307,7 +307,7 @@ backtrace_print(const struct backtrace *bt, int fd)
 		int chars_written = dprintf(fd, C_FRAME_STR_FMT "\n", frame_no,
 					    frame->ip, proc_name, offset);
 		if (chars_written < 0) {
-			say_error("unwinding error: dprintf failed: %s",
+			say_debug("unwinding error: dprintf failed: %s",
 				  tt_strerror(errno));
 			break;
 		}