Fix for #72 - Using SPEL as rule value caused incorrect argument 1
replacement in error message.
Implementation of #62 - Allowing user to specify a string-literal error
message instead of a message code lookup
Implementation of #73 - Allowing the use of SPEL expressions within
error messages, populated from current SPEL context. If SPEL message
arguments or SPEL expressions resolve _into_ SPEL, they are stripped of
their SPEL ${ notation to prevent malicious SPEL injection.