Allow LanguageRuntimes to return an error if they fail in the course of dynamic type discovery

This is not meant to report that a value doesn't have a dynamic type - it is only meant as a mechanism to propagate actual type discovery issues (e.g. malformed type metadata for languages that have such a notion)

This information is used by ValueObjectDynamic to set its own m_error, which is a fairly sharp and heavyweight tool to begin with

For the time being, this is an architectural improvement but a practical no-op as no existing runtimes are actually setting errors

llvm-svn: 268591