Help users identify missing symbols by adding some logging when symbols can't be resolved

[MarioHewardt]

Expected behavior

When frames in a restrack callstack can't be resolved, output a log statement indicating that it might be as a result of missing symbols.

Actual behavior

No symbols/empty callstack/frames.

Steps to reproduce the behavior

1. Use the -restrack switch targeting an app without symbols accessible.
2. The resulting restrack output shows frames with no symbolic information

System information (e.g., distro, kernel version, etc.)

Linux (all)