CustomException Class for adding already existing entries

[ftjahn8]

Current Situation:
If an already existing entry is added again to the KeePass, it raises an Exception (no custom Subclass, the base Exception!).

Example Traceback / Code line for potential change

File "XXXXX\venv\lib\site-packages\pykeepass\pykeepass.py", line 613, in add_entry
    raise Exception(
Exception: An entry "XXXXX" already exists in "Group: "XXXXX""

Wanted Behaviour:
A custom Exception is thrown by the code at this point.

Reason:

1. Prevent the need to catch bare Exceptions to identify other occuring exceptions during (this or other) actions.
2. Good style (PyLint Warning: W0703: Catching too general exception Exception (broad-except) for try: except Exception:)

Best Regards, ftjahn8