Generate default error message

[PhilipIngham]

Overview

I have added a default error message for the Assertions.fail() function as discussed in the issue, and made some grammatical changes in the surrounding areas. As said in the issue, it's a very minor problem, so nothing is really lost if this doesn't get implemented, but the option's there.

I hereby agree to the terms of the JUnit Contributor License Agreement.

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes