Insufficient error message when passing invalid argument to -f option of export command

[515hikaru]

• [x] I have searched the issues of this repo and believe that this is not a duplicate.

Issue

Though this issue is similar to python-poetry/poetry#1640, different a bit.I think the Exporter error message is not enough.

When I used export command for the first time, I didn't know how to specify -f option. So I tried:

$ poetry export -f requirements -o requirements.txt 

The result of the above command is below.

[ValueError]
Invalid export format: requirements

poetry said I was wrong, but it did not tell me how should I do. The error message should tell us how to specify -f option. For example, if it show list of accepted formats, it helping for me.