Don't create single-letter symbols for "export" error messages

There's no reason to call getGlobalSymbol since we're not going to end
up exporting a symbol.

Author: d-torrance

M2/Macaulay2/m2/packages.m2

@@ -428,7 +428,7 @@ export List   := v -> (
 		if class sym#1 =!= String then error("expected a string: ", nam);
 		sym = getGlobalSymbol(pd, sym#1))
 	    else if instance(sym, String) then (
-		if match("^[[:alpha:]]$", sym) then error ("cannot export single-letter symbol ", getGlobalSymbol(pd, sym));
+		if match("^[[:alpha:]]$", sym) then error ("cannot export single-letter symbol '", sym, "'");
 		nam = sym;
 		sym = if pd#?nam then pd#nam else getGlobalSymbol(pd, nam))
 	    else error ("'export' expected a string or an option but was given ", sym, ", of class ", class sym);