patch 8.1.1863: confusing error when using a builtin function as method

Problem:    Confusing error when using a builtin function as method while it
            does not support that.
Solution:   Add a specific error message.
diff --git a/src/userfunc.c b/src/userfunc.c
index 2bdc2b1..fd25090 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -1678,6 +1678,11 @@
 	    case ERROR_UNKNOWN:
 		    emsg_funcname(N_("E117: Unknown function: %s"), name);
 		    break;
+	    case ERROR_NOTMETHOD:
+		    emsg_funcname(
+			       N_("E276: Cannot use function as a method: %s"),
+									 name);
+		    break;
 	    case ERROR_DELETED:
 		    emsg_funcname(N_("E933: Function was deleted: %s"), name);
 		    break;
@@ -1685,15 +1690,18 @@
 		    emsg_funcname((char *)e_toomanyarg, name);
 		    break;
 	    case ERROR_TOOFEW:
-		    emsg_funcname(N_("E119: Not enough arguments for function: %s"),
+		    emsg_funcname(
+			     N_("E119: Not enough arguments for function: %s"),
 									name);
 		    break;
 	    case ERROR_SCRIPT:
-		    emsg_funcname(N_("E120: Using <SID> not in a script context: %s"),
+		    emsg_funcname(
+			   N_("E120: Using <SID> not in a script context: %s"),
 									name);
 		    break;
 	    case ERROR_DICT:
-		    emsg_funcname(N_("E725: Calling dict function without Dictionary: %s"),
+		    emsg_funcname(
+		      N_("E725: Calling dict function without Dictionary: %s"),
 									name);
 		    break;
 	}