fix: use error return trace in senderror

This should produce more usable error messages. The error messages before
this were pointing at zap runtime functions, instead of at the code that
produced the error.

examples/senderror/senderror.zig

@@ -7,7 +7,7 @@ fn MAKE_MEGA_ERROR() !void {
 
 fn MY_REQUEST_HANDLER(r: zap.Request) void {
     MAKE_MEGA_ERROR() catch |err| {
-        r.sendError(err, 505);
+        r.sendError(err, if (@errorReturnTrace()) |t| t.* else null, 505);
     };
 }
 

src/request.zig

@@ -319,17 +319,17 @@ pub fn getUserContext(self: *const Self, comptime Context: type) ?*Context {
 }
 
 /// Tries to send an error stack trace.
-pub fn sendError(self: *const Self, err: anyerror, errorcode_num: usize) void {
+pub fn sendError(self: *const Self, err: anyerror, err_trace: ?std.builtin.StackTrace, errorcode_num: usize) void {
     // TODO: query accept headers
-    if (self._internal_sendError(err, errorcode_num)) {
+    if (self._internal_sendError(err, err_trace, errorcode_num)) {
         return;
     } else |_| {
         self.sendBody(@errorName(err)) catch return;
     }
 }
 
 /// Used internally. Probably does not need to be public.
-pub fn _internal_sendError(self: *const Self, err: anyerror, errorcode_num: usize) !void {
+pub fn _internal_sendError(self: *const Self, err: anyerror, err_trace: ?std.builtin.StackTrace, errorcode_num: usize) !void {
     // TODO: query accept headers
     // TODO: let's hope 20k is enough. Maybe just really allocate here
     self.h.*.status = errorcode_num;
@@ -339,9 +339,12 @@ pub fn _internal_sendError(self: *const Self, err: anyerror, errorcode_num: usiz
     var writer = string.writer();
     try writer.print("ERROR: {any}\n\n", .{err});
 
-    const debugInfo = try std.debug.getSelfDebugInfo();
-    const ttyConfig: std.io.tty.Config = .no_color;
-    try std.debug.writeCurrentStackTrace(writer, debugInfo, ttyConfig, null);
+    if (err_trace) |trace| {
+        const debugInfo = try std.debug.getSelfDebugInfo();
+        const ttyConfig: std.io.tty.Config = .no_color;
+        try std.debug.writeStackTrace(trace, writer, fba.allocator(), debugInfo, ttyConfig);
+    }
+
     try self.sendBody(string.items);
 }