[lldb comparator] raise error if executable is not found

[Vipul-Cariappa]

Example

❯ idd -c lldb -ba ../../tmp/idd-demo/fib_loo -ra ../../tmp/idd-demo/fib_tail_cal
Traceback (most recent call last):
  File "/workspaces/cpp-py/opt/idd/idd.py", line 362, in <module>
    Debugger = LLDBDebugger(ba, ra)
  File "/workspaces/cpp-py/opt/idd/debuggers/lldb/lldb_driver.py", line 33, in __init__
    raise Exception(base_error.GetCString())
Exception: unable to find executable for '../../tmp/idd-demo/fib_loo'