Better error messages when PY_PYTHON* is malformed

[brettcannon]

Closes #108

For PY_PYTHON3="3.8.10" without PYLAUNCH_DEBUG:

Error parsing '8.10' as an integer: invalid digit found in string

With PYLAUNCH_DEBUG:

Checking the PY_PYTHON3 environment variable
PY_PYTHON3 = '3.8.10'
Error parsing '8.10' as an integer: invalid digit found in string

[brettcannon]

The coverage failures are false-positives.

[jefftriplett]

Would this show up in verbose (-v) mode too?

[brettcannon]

Would this show up in verbose (-v) mode too?

No, that's passed directly to Python and I don't want to inject the Python Launcher into that as people parse that output.

[jefftriplett]

That makes sense. I guess I rarely if ever run python -v. I think the error message is much clearer about what is going on. It's much easier to solve a problem when it's obvious why something is failing. 👍🏼