Allow .error_class argument to be class or error condition

[egnha]

This enhancement concerns the .error_class argument of firmly(). It would enable the creation of more nuanced error condition objects that could be used to:

• Embed validation-failure messages, individually, for further handling down the call stack (#31)
• Modify the call expression via a functional parameter
• Enable more informative call expression for validate() (#36)

Accordingly, rename .error_class to .error, and set its default value to NULL (which would mean using "simpleError" as condition subclass and the current call-expression transformation (#33)).

[egnha]

Implemented in quosures branch in commit 3160be7c472edd80844e21fa16d4ff1418248adc.