GH-125873: error out in getpath when unable to find stdlib

[FFY00]

• Issue: Show a better error message when PYTHONHOME is wrong #125873

[nineteendo]

Could you show an example?

[FFY00]

Yes, sorry.

$ PYTHONHOME=inexistent python3.14td
Failed to find the standard library directory (stdlib_dir='inexistent/lib/python3.14t', stdlib_zip='inexistent/lib/python314t.zip')
Failed to find the platform-specifc standard library directory (platstdlib_dir='inexistent/lib/python3.14t/lib-dynload')

Hint
| The home directory was manually set to 'inexistent' via the PYTHONHOME environment variable.
| This value might point to an invalid Python home directory.

Fatal Python error: error evaluating path
Python runtime state: core initialized

Current thread 0x0000783aacb54740 (most recent call first):
  <no Python frame>