lei_input: provide hint for bare "L:" and "kw:"

I just made this mistake running "lei import" myself, so
I figure giving a hint makes sense, here.

1 files changed, 4 insertions, 1 deletions

diff --git a/lib/PublicInbox/LeiInput.pm b/lib/PublicInbox/LeiInput.pm
index cb71e97c..8ce445c8 100644
--- a/lib/PublicInbox/LeiInput.pm
+++ b/lib/PublicInbox/LeiInput.pm
@@ -276,7 +276,10 @@ sub prepare_inputs { # returns undef on error
                                 $may_sync and $input = 'maildir:'.
                                                 $lei->abs_path($input_path);
                         } else {
-                                return $lei->fail("Unable to handle $input");
+                                my $m = "Unable to handle $input";
+                                $input =~ /\A(?:L|kw):/ and
+                                        $m .= ", did you mean +$input?";
+                                return $lei->fail($m);
                         }
                 } elsif ($input =~ /\.(?:eml|patch)\z/i && -f $input) {
                         lc($in_fmt//'eml') eq 'eml' or return $lei->fail(<<"");