MINOR: spoe: Add "option set-on-error" statement

MINOR: spoe: Add "option set-on-error" statement

It defines the variable to set when an error occurred during an event
processing. It will only be set when an error occurred in the scope of the
transaction. As for all other variables define by the SPOE, it will be
prefixed. So, if your variable name is "error" and your prefix is "my_spoe_pfx",
the variable will be "txn.my_spoe_pfx.error".

When set, the variable is the boolean "true". Note that if "option
continue-on-error" is set, the variable is not automatically removed between
events processing.