Add user-friendly message when missing importer

[edoardob90]

Fixes #1486.

Add a clear error message if a user tries to import a non-jrnl file, since that's the only importer available right now.

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed.

Note: I had to make some changes in line with PR #1955 to have passing tests via tox. Everything's fine with pytest because of the pinned version of pytest-bdd==7.3.0.

[edoardob90]

A collateral comment about an implementation detail I came across with this fix: the jrnl.messages.MsgText class cannot be sub-classed. I could see an advantage of adjusting it when one wants to add custom messages for a new importer, for example.

This change would be rather straightforward:

• Create a MsgTextBase(Enum)
• Have MsgText inherits from it
• Adjust the __init__.py imports
• Optional: fix the problems with typing related to the messages package

Instead of

from jrnl.messages import Message
# and similar lines

do

from typing import TypeAlias

from jrnl.messages import Message as _Message
# other imports

Message: TypeAlias = _Message.Message
# other lines