fix: report import errors from coder runs

[Kcstring]

Summary

• catch ImportError around coder.run() entry points in main()
• report dependency/import failures as user-facing errors with the existing install docs link instead of crashing with a traceback
• add a regression test for the --message path

Testing

• python3 -m pytest tests/basic/test_main.py -k "message_import_error_is_reported or main_message_adds_to_input_history"
• python3 -m ruff check aider/main.py tests/basic/test_main.py

Fixes #5051