fix: correct 'last error' to 'last layer' in ValueError message

[Jah-yee]

Summary

Fix a typo in the error message emitted by check_Vs_profile_format():
"last error" → "last layer".

Changes

• PySeismoSoil/helper_generic.py: Changed the ValueError message for invalid
  material numbers from except for the last error to except for the last layer, consistent with the thickness check above it.

• tests/test_class_Vs_profile.py: Updated test_Vs_profile_format__case_10
  to match the full error message (preventing future regressions on this
  specific wording).

Testing

The fix was verified by running the reproduction case from issue #121:

import numpy as np
from PySeismoSoil.class_Vs_profile import Vs_Profile

data = np.array(
    [[10, 20, 30, 0], [100, 120, 160, 190],
     [0.01, 0.01, 0.01, 0.01], [1600, 1600, 1600, 1600], [1, 0, 3, 0]],
    dtype=float,
).T
Vs_Profile(data)
# Before: ValueError: The "material number" column should be all positive, except for the last error.
# After:  ValueError: The "material number" column should be all positive, except for the last layer.

Closes #121

[Wkhanex]

Verified the corrected diagnostic with a synthetic two-layer, five-column profile: a zero material number in the first layer raises The "material number" column should be all positive, except for the last layer. A zero material number in the final layer is accepted, so the corrected wording matches the actual exception rule. I ran these checks using the extracted complete validator and its numeric helpers on Python 3.12/NumPy; the full library, tox and example notebooks were not run, and this is not a merge approval.