BUG/MINOR: spoe: Register the variable to set when an error occurred

BUG/MINOR: spoe: Register the variable to set when an error occurred

Variables referenced in HAProxy's configuration file are registered during the
configuration parsing (during parsing of "var", "set-var" or "unset-var"
keywords). For the SPOE, you can use "register-var-names" directive to
explicitly register variable names. All unknown variables will be rejected
(unless you set "force-set-var" option). But, the variable set when an error
occurred (when "set-on-error" option is defined) should also be regiestered by
default. This is done with this patch.