fix: prefix messages with 'asdf: '

[hyperupcall]

Summary

Sometimes asdf prints messages, and it's not obvious that the message is coming from asdf. This fixes that.

It does so by prepending asdf: to every single user-read console message, but only on the asdf exec subcommand codepath.

Fixes: #1467

[jthegedus]

I am not sure this is the right solution to this problem. The initial report has a few things going on:

1. .tool-versions wasn't interpolated
2. the user found the error output format confusing
3. the user did not know which tool produced this error

I don't think prefixing all outputs with asdf: is the best solution here.

I do like the OPs suggestion for including

asdf did not find a version...

That is clear which tool produced the error.

And so perhaps this PR can be used to make that change.

That solves 3.

Solving 1 will take more investigation.

Solving 2 I think is related to #928 more than anything.

[hyperupcall]

Yeah you're right, that's a lot better - I'm not sure what I was thinking...