silence warnings from dangling asdf shims (#110)
silence warnings from dangling asdf shims (#110)

If you are using asdf to manage python versions and don't have, say, 3.9 installed+configured, 
each run of pants will show an error like:

```
No preset version installed for command python3.9
...
```

The script will continue on, but the error message is confusing.

This change simplifies a check for a similar case with pyenv to just ignore broken interpreter-ish files 
in general, including dangling pyenv and asdf shims.


jsirois/setup,pantsbuild/setup,jsirois/setup,pantsbuild/setup,jsirois/setup,pantsbuild/setup