fix: improve readability

[michen00]

This may be a nit, but here is why I believe these changes improve readability:

• Clarity: Adding “a” (“assign to a variable first”) fixes the grammar and reads more naturally. Splitting with “;” makes the guidance easier to scan as two parts: the rule and the action.
• Consistency: EM101–EM103 now match the plural form already used in EM104 and EM105 (“Exceptions must not…”), which is more consistent across rules.
• Style: The semicolon is clearer than the prior comma splice.

As for verbosity, the messages are slightly longer, but still concise for CLI output. Specificity remains unchanged and the directive remains clear at a glance.

[Copilot]

Pull Request Overview

This PR improves the grammar and readability of error messages in a flake8 plugin by making them more consistent and grammatically correct.

• Updates error messages EM101-EM103 to use plural "Exceptions" for consistency with EM104-EM105
• Replaces comma splices with semicolons for better grammar and clarity
• Adds the article "a" to improve natural language flow

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/flake8_errmsg/init.py	Updates error message strings for EM101, EM102, and EM103 with improved grammar and consistency
tests/test_package.py	Updates test assertions to match the new error message formats

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[michen00]

note on line length: I split at 88, but I notice long lines in other spots. I'm happy to make an additional style commit; just lmk :)

[henryiii]

Only issue I had was with the title. Only the strictest SemVer would call this a feature. Python is EffVer. :)