Verify whether `lldb` or `gdb` are available (instead of bluntly failing)

[bittner]

The current implementation assumes that lldb (on macOS) or gdb (on GNU/Linux) are installed. Thus, when they are not, execution fails with a stack trace and a rather cryptic error message.

It would be nice if django-sql-sniffer would instead verify whether the binary is available for execution and abort gracefully, ideally giving hints on how to resolve the situation.

Suggested Solution

$ django-sql-sniffer -p 12345 -t
ERROR: gdb, the GNU debugger, appears to be missing on your system. django-sql-sniffer relies on this tool.
Try `sudo apt-get install gdb` or `sudo dnf install gdb` to fix this problem. ABORTING.