fix(gnolang): make Go2Gno return a prespective error instead of sudde…

…n/elusive runtime panic with a bad receiver (#3733)

The pattern:

```go
func() A()
```
confuses Go into expecting a receiver and it returns a compile time
error "missing receiver", but previously Gno panicked with a runtime
error due to a deference yet the Recv.List was empty. This change fixes
that by detecting that condition and prescriptively panicking which can
then be relayed reasonably as expecting to the calling user.

Fixes #3727

---------

Co-authored-by: Morgan Bazalgette <[email protected]>

gnovm/pkg/gnolang/go2gno.go

@@ -433,7 +433,10 @@ func Go2Gno(fs *token.FileSet, gon ast.Node) (n Node) {
 		recv := FieldTypeExpr{}
 		if isMethod {
 			if len(gon.Recv.List) > 1 {
-				panic("*ast.FuncDecl cannot have multiple receivers")
+				panic("method has multiple receivers")
+			}
+			if len(gon.Recv.List) == 0 {
+				panic("method has no receiver")
 			}
 			recv = *Go2Gno(fs, gon.Recv.List[0]).(*FieldTypeExpr)
 		}

gnovm/tests/files/parse_err0.gno

@@ -0,0 +1,10 @@
+// https://github.com/gnolang/gno/issues/3727
+
+package main
+
+func () A()
+
+func main() {}
+
+// Error:
+// method has no receiver