Add FailureMessage.Err method and AssertionError type

[lycis]

Why

Sometimes you want to get an error from an assertion rather than directly fail a test run. Also test suites like Godog rely on returning an error instead of directly failing tests (e.g. because they encapsulate direct access to the test instance).

What

The FailureMessage type received a function AsError() that returns an instance of error or nil with the according failure message.

Checklist

• CHANGELOG.md is updated.
• README.md is updated.
• The code changes follow Effective Go.
• The code changes follow CodeReviewComments.
• The code changes are covered by tests.

[lycis]

I like the suggest changes. I implemented them by:

• Introducing the error-compatible type AssertionError
• Refactoring AsError() to Err() to be more idiomatic.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (7b197d1) to head (c02f19d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #141   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        16    +1     
  Lines          438       444    +6     
=========================================
+ Hits           438       444    +6     

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[pellared]

Thanks for your contribution. I plan to make a release today.