Errors need to be classes instead of interfaces so that they can be used as values (e.g. with instanceof)

[heikkipora]

Example code snippet that did work on earlier spex versions but does not work with current release:

import spex from 'spex'
const {BatchError} = spex.errors

..

if (e instanceof BatchError) {
  // proceed to do stuff with e.getErrors()
}

This PR will revert the relevant changes made in d9d40f9#diff-ed54ac027337bce297669fa4cac590aa3fd86567cd32bf84d8fb48881e27d9f7

[vitaly-t]

@heikkipora Thanks! I have released v4.1.0 with this fix. Plus, pg-promise v12.3.0 now uses it.

[heikkipora]

Thanks for a quick reaction & release!