Ensure download location exists before listing installed terraform versions

[mmourick]

What

Ensures download location exists before listing installed terraform versions.

Why

To prevent error listing installation directory from happening when an user lists installed terraform versions before having downloaded any.

References

• Closes tfversion list --installed produces an error on clean installation. #11

[bschaatsbergen]

If you try to use the command $ tfversion list --installed on a fresh machine, should it show an error? Wouldn't it be better if it just gives back an empty list because nothing is installed? Is it necessary to show an error message in this situation?

[mmourick]

@bschaatsbergen What this pr does is prevent an error being thrown as a result of the download directory not yet being created.

The following no longer occurs on a fresh installation:

tfversion list --installed
error listing installation directory: open /Users/mathijs/.tfversion: no such file or directory

However, with this fix in place, the user is still presented with this error:

tfversion list --installed
error listing installed versions: no versions found

This error message already is much more clear to the user and probably was meant to be displayed to the user in this scenario, so it's definitely an improvement.
However, I agree it does not warrant an error message. If you want I can update that behavior as well in this PR. I had left it out since I figured the above was the desired behavior.

[ChrisTerBeke]

Ensuring the location exists in the GetDownloadLocation() method seems like the right solution to me. What do you think @bschaatsbergen?