patch 8.2.2846: Vim9: "echo Func()" does not give an error for using void

Problem:    Vim9: "echo Func()" does not give an error for a function without
            a return value.
Solution:   Give an error.  Be more specific about why a value is invalid.
diff --git a/src/typval.c b/src/typval.c
index 9ca3569..3d1bd7f 100644
--- a/src/typval.c
+++ b/src/typval.c
@@ -522,7 +522,8 @@
 	case VAR_ANY:
 	case VAR_VOID:
 	case VAR_INSTR:
-	    emsg(_(e_inval_string));
+	    semsg(_(e_using_invalid_value_as_string_str),
+						  vartype_name(varp->v_type));
 	    break;
     }
     return NULL;