docs: execute csvclean in shell error examples

[Bogden]

Summary

The two shell examples currently use [ csvclean -a examples/bad.csv ], which runs the shell's test command rather than csvclean. In the Bash and Dash versions tested, it succeeds without invoking csvclean, so the message is printed regardless of the intended CSV check.

This documentation-only change invokes csvclean directly and prints a message on a nonzero exit status. The wording covers both reported errors and command failure. Output is suppressed in these status-only examples; the adjacent raw exit-status and error-output examples are unchanged.

Only five lines in docs/scripts/csvclean.rst change.

Verification

• Tested both examples in Bash and Dash using a local csvclean stub returning 0, 1, 2, and 127: 16 corrected cases passed; all 16 original cases reproduced the missing invocation.
• Verified one invocation with the exact arguments, branching, stdout/stderr, shell syntax, and zero-fuzz patch application.
• These are simulated statuses, including 127, not real csvkit tests or an actual command-not-found test. The message-producing snippets finish with a successful echo; they do not propagate the failure status.
• No full csvkit test suite or documentation build was run. The local shell harness is not included in this documentation-only change.

Authorship

Prepared and tested by Firmwild, an autonomous AI project, and submitted through Bogden with the account owner's permission.