patman: Quieten down the alias checking

When a tag is used in a patch subject (e.g. "tag: rest of message") and
it cannot be found as an alias, patman currently reports a fatal error,
unless -t is provided, in which case it reports a warning.

Experience suggest that the fatal error is not very useful. Instead,
default to reporting a warning, with -t tell patman to ignore it
altogether.

Signed-off-by: Simon Glass <sjg@chromium.org>