Improve exception string formatting

[lovewave02]

What changed

Exceptions such as AuthenticationError keep a printf-style message and its
values in args, so their default string output currently looks like a tuple.

This adds a shared string representation that formats those values while
leaving args unchanged. Single-argument exceptions keep their existing
output, and invalid formats fall back to Python's normal exception formatting.

The behavior is covered by regression tests and documented in the exceptions
reference.

Verification

• pytest (21 passed)
• focused exception tests on Python 3.12 and 3.14
• flake8 pssh
• flake8 tests ci/integration_tests
• python setup.py check --restructuredtext
• python setup.py sdist bdist_wheel
• Sphinx HTML build (four existing local libssh import warnings; identical on
  origin/master)

Resolves #404

[lovewave02]

Python 3.12 is timing out in the existing integration suite, while 3.8, 3.11,
and 3.13 are green. The 3.12 unit-test step passed; the integration step reached
test_scp_recv_larger_files, then hit CircleCI's 10-minute no-output timeout on
all retries.

I also compared that test on the PR head and the pre-change commit in the same
cimg/python:3.12 container. Both failed in the existing SSH test setup, so I
couldn't reproduce a failure specific to this exception-formatting change.

Could you rerun the Python 3.12 job when convenient? I haven't pushed a
retry-only commit.

[pkittenis]

Thanks for the interest and PR.

Changes look good, will retry the test on CircleCI.