Remove explicit instantiations of ErrorDescription values

[pcapriotti]

So far, every ErrorDescription type has been accompanied by a corresponding value created with mkErrorDescription. This PR removes all those definitions, and adds some convenience functions (to be used with type application) that make it easier to refer to the error without having an explicit typed instantiation available.

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• changelog.d contains the following bits of information:
  • A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.

[fisx]

nice!

[fisx]

should this be hidden from export? we don't want to use it any more outside of this module, right?

[pcapriotti]

Some errors are parametrised, and in that case we need to use this function to convert them to Wai.Error, because mkErrorDescription is not sufficient.