Invalid option->default() does not prettify exceptions

[emanb29]

When a value passed to default() does not pass a must() closure, the output should be formatted in the standard prettyprint. However, the current behavior is to throw an ugly exception and stacktrace.

[NeoVance]

The user creating the command should validate the value being supplied to default is an acceptable default. While this type of error could be caught, it should be left up to the user to handle this type of error, since this is not caused by end user input but a mistake made by the programmer using the library.

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