Clean up and clarify semantic_error chaining

Clean up and clarify semantic_error chaining

I consolidated the copy-construction that every target_symbol::chain
caller was doing.  I also removed the comments that target_symbol errors
might be chained due to function wildcards, because it's not true --
each derived instance gets a deep_copy of the probe body.  However,
@cast can still chain since it may try multiple modules to resolve the
casting dereference.