FIXED: if a previous handler returns completion, a pull_response can
find a died pengine.  We should not consider this an error.
ADDED: Formal error code to error objects, making it easier to act on
them from the program.
FIXED: non-determinism in http create
ADDED: Error context to error events (formal code and location when available).
ADDED: Context information for output that originates from print_message/2.