Errors in collecting stack traces can result in empty string

[danobi]

What reproduces the bug? Provide code if possible.

Currently the following output is possible:

^C
...
@[]: 1234

This is incredibly misleading and hard to diagnose. What's worse, the error LOG is printed before any map output, so it gets immediately lost in the scrollback.

The fix would be to show better diagnostics in applicable cases. See #2392 for full discussion