st: Minor update to -jmp- on SSC

["Jann, Ben" <ben.jann@soz.gess.ethz.ch>]

Thanks to Kit Baum, the -jmp- package has 
been updated on SSC.

-jmp- issued a cryptic error message when used after 
some estimation commands (e.g. -heckman-). The error 
message is more informative now. 

ben

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