Error message should be given when .pyenv folder already exists

[otolabqu]

While following the instructions to curl -L https://github.com/pyenv/pyenv-installer/raw/master/bin/pyenv-installer | bash not much happened, but no error message was given either.

This was because the .pyenv folder already existed (due to a previous failed installation attempt).

If the script aborts due to the .pyenv folder already existing, it would be useful with a warning or error message telling this.

I got past this by removing the .pyenv folder and then running the above again, which then installed as it should.

[shubhodeep9]

I am happy to help, if the need be.

[obestwalter]

Thanks @otolabqu and @shubhodeep9 - a PR implementing this would likely be accepted.

[shubhodeep9]

@obestwalter I will work on it.

[shubhodeep9]

@obestwalter I have created a PR to this #82