Keep PathAccessError printable for Scope/S paths (#249)

[gaoflow]

Fixes #249.

A PathAccessError raised from a Scope/S access cannot stringify itself — str(exc) renders as <exception str() failed>:

import glom
glom.glom({}, glom.S['X'], scope={'x': 'y'})

glom.core.PathAccessError: <exception str() failed>

Cause

PathAccessError.get_message() does Path(self.path).values()[self.part_idx]. When the access originates from a scope, self.path is not a plain T-based path, and Path(...) raises ValueError: path segment must be path from T, not S. Because get_message() is called from GlomError.__str__, that exception escapes and the error can no longer be printed (and get_message() itself crashes).

Fix

Guard the Path(self.path) lookup and fall back to self.path when it can't be turned into a plain path, so the message stays readable instead of raising:

glom.core.PathAccessError: could not access T['X'], part 0 of T['X'], got error: KeyError('X')

An exception should always be able to produce a string and never crash its own formatter, so this restores the documented "readable error message" contract for the S/scope case without changing the normal T-path output.

Tests

Added test_pae_scope_printable in glom/test/test_error.py, asserting the scope-originated PathAccessError is printable (no <exception str() failed>) and that get_message() returns a string. It is RED without the fix and GREEN with it. The error and scope suites pass (test_error.py, test_scope_vars.py: 27 passed); the only failing tests in the repo are pre-existing test_cli.py environment failures unrelated to this change.

Disclosure: I prepared this fix with AI assistance under my direction; I reviewed and verified the change and the test myself.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.22%. Comparing base (64141ba) to head (7583a20).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #298      +/-   ##
==========================================
- Coverage   98.25%   98.22%   -0.04%     
==========================================
  Files          27       27              
  Lines        4364     4384      +20     
  Branches      612      612              
==========================================
+ Hits         4288     4306      +18     
- Misses         52       53       +1     
- Partials       24       25       +1     

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[mahmoud]

This is a really good catch. I think I found a slightly simpler approach since all .path attribute accesses are actually Path instances. Wish I had typing to enforce this, but in the meantime I put a fallback in there. Pushed the adapted fix to the branch here and looks good now. Thanks a bunch!