portablectl: quote the profile name in the validation error

With an empty profile we get:

	$ portablectl attach -p "" foo
	Unit profile name not valid:

It's not clear what is the profile. Let's quote it so it's clear also
for empty profile names.

(cherry picked from commit 6c8b21d30db4f4d5a0cdce9f2dc6a99b9b47e7c7)

src/portable/portablectl.c

@@ -1590,7 +1590,7 @@ static int parse_argv(int argc, char *argv[], char ***remaining_args) {
 
                         if (!filename_is_valid(opts.arg))
                                 return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
-                                                       "Unit profile name not valid: %s", opts.arg);
+                                                       "Profile name '%s' is not valid.", opts.arg);
 
                         arg_profile = opts.arg;
                         break;