fix(gnovm): improve error message for nil assignment in variable declaration

gnovm/pkg/gnolang/type_check.go

@@ -289,7 +289,7 @@ func checkAssignableTo(xt, dt Type, autoNative bool) error {
 	// case0
 	if xt == nil { // see test/files/types/eql_0f18
 		if !maybeNil(dt) {
-			panic(fmt.Sprintf("invalid operation, nil can not be compared to %v", dt))
+			panic(fmt.Sprintf("cannot use nil as %v value in variable declaration", dt))
 		}
 		return nil
 	} else if dt == nil { // _ = xxx, assign8.gno, 0f31. else cases?

gnovm/tests/files/assign29.gno

@@ -0,0 +1,10 @@
+package main
+
+func main() {
+	a := 1
+	a = nil
+	println(a)
+}
+
+// Error:
+// main/files/assign29.gno:5:2: cannot use nil as int value in variable declaration

gnovm/tests/files/var31.gno

@@ -0,0 +1,8 @@
+package main
+
+func main() {
+	var i int = nil
+}
+
+// Error:
+// main/files/var31.gno:4:6: cannot use nil as int value in variable declaration