Update RascalAmendmentProposals.md

Author: jurgenvinju

courses/RascalAmendmentProposals/RascalAmendmentProposals.md

@@ -30,4 +30,5 @@ not be turned into actual maintenance projects on the language. Completed RAPs a
 * [ ]  ((RAP13)) - Name-parametrized syntax modifiers
 * [ ]  ((RAP14)) - Backward compatibility for Rascal modules
 * [ ]  ((RAP15)) - Conditional patterns and removal of accidental non-linear matching
+* [ ]  ((RAP16)) - Support for error trees