Add "attempt" keyword for go-like error handling

Returns a pair (value, err)

If code evaluates successfully, then err is null.  Otherwise, it's an
Error object with information about the error that occurred.

Author: d-torrance

M2/Macaulay2/d/binding.d

@@ -318,6 +318,7 @@ bumpPrecedence();
      export breakpointS    := special("breakpoint",    unaryop, precSpace, wide);
      export profileS       := special("profile",       unaryop, precSpace, wide);
      export shieldS        := special("shield",        unaryop, precSpace, wide);
+     export attemptS       := special("attempt",       unaryop, precSpace, wide);
      export throwS         := special("throw",        nunaryop, precSpace, wide);
      export returnS        := special("return",       nunaryop, precSpace, wide);
      export breakS         := special("break",        nunaryop, precSpace, wide);

M2/Macaulay2/d/evaluate.d

@@ -1677,6 +1677,16 @@ export toExpr(err:Error):Expr := (
     storeInHashTable(h, toExpr("message"), toExpr(err.message));
     Expr(sethash(h, false)));
 
+attemptfun(c:Code):Expr := (
+    r := tryEval(c);
+    if tryEvalSuccess then seq(r, nullE)
+    else (
+	when r
+	is err:Error do seq(nullE, toExpr(err))
+	-- shouldn't happen since tryEvalSuccess only false when r is an Error
+	else buildErrorPacket("unable to catch error")));
+setupop(attemptS, attemptfun);
+
 assigntofun(lhs:Code,rhs:Code):Expr := (
     left := eval(lhs);
     when left is Error do return left else (

M2/Macaulay2/m2/exports.m2

@@ -514,6 +514,7 @@ export {
 	"atan",
 	"atan2",
 	"atanh",
+	"attempt",
 	"autoload",
 	"baseFilename",
 	"baseName",