fix: properly handle nested unresolvable type during mapping

[romm]

This change brings two enhancements:

• When mapping to a structure that contains a nested unresolvable type, for instance a doc-block type that does not match the native type, the mapper will detect it and throw a proper exception (it used to just crash on a badly designed assertion).
• Unresolvable types that are not used during mapping, for instance parameters of methods that are not used by the mapper will no longer throw an exception even if not needed by the library. Instead, an unresolvable type is assigned to these invalid properties/parameters/ return types.