Improve interpreter identification failure logging

[jsirois]

Currently, when a binary that looks like it might be a python binary is executed to determine the potential python binary's identity, failures from valid but misconfigured python binaries are more opaque than is useful to debug the underlying misconfiguration:
https://github.com/pantsbuild/pex/blob/e906d4d6430071dbe7269a13b48641979d60c005/pex/interpreter.py#L348-L364

In particular, except Exception as e drops details on the floor in the case where the exception is a pex.executor.Executor.NonZeroExit since its __str__ does not include its stdout or stderr fields. Make sure these are logged.

[jsirois]

It looks like this was all fixed up though a series of changes culminating in #1500. In short, interpreter identification is now done via the Job system and Job errors are retained and printed. These job errors include the stderr from the Job, which provides good detail on what the issue was identifying the interpreter.