Fix matrix sparsity analysis input handling

[fallintoplace]

Summary

• Move the Matrix Market CSR conversion back onto the valid-input path.
• Return a nonzero exit code and write usage errors to stderr when no matrix path is provided.

Why

analyse_mtx_sparsity.py only assigned M inside the if M_in is None branch, after a raise. For valid inputs, the first M.has_sorted_indices access raised NameError before any sparsity analysis could run.

Validation

• python3 -m py_compile python/utils/analyse_mtx_sparsity.py
• No-argument invocation exits with status 1 and prints the usage error.
• Smoke-tested a tiny temporary Matrix Market file with NumPy, SciPy, NetworkX 2.8.8, and Matplotlib.

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[ChuckHastings]

/okay to test f06564e

[ChuckHastings]

/merge