Catch Exceptions by Reference to Avoid Slicing

[ahcox]

Preserve messages from derived exceptions for more informative logging.

Catching them by value means making a copy of the thrown exception, and if it was actually derived from the type named at the catch point, that will be a slicing copy, throwing away the fields of the derived type which might contain information it uses to provide a helpful what() message. Indeed, the virtual what() function will resolve to the base class version, returning an empty string in the cases of the test apps.

Ref: C++ Core Guidelines, E.15: Throw by value, catch exceptions from a hierarchy by reference.

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