refactor: make descriptors::key::DescriptorKeyParseError an enum

refactor: add MalformedKeyDataKind enum to DescriptorKeyParseError definition

fix: re-export MalformedKeyDataKind from descriptor module root

fix: allow missing docs on C-style MalformedKeyDataKind enum

fix: start error strings with lowercase in descriptors::key module

Author: chris-ricketts