Don't return arbitrary Python exception text to remote clients

[oseasandrepro]

You currently do:

response_payload = str(e)

This can leak details such as:

/key/internal/database.py
SQL statement...
credentials/configuration details...
internal object names...

For an experimental framework it's understandable, but protocol-wise it should become something like:

status = PROCEDURE_EXECUTION_ERROR
message = "Remote procedure execution failed"

while the complete exception stays server-side:

self.__logger.exception(...)

Possibly expose detailed errors only through a debug mode