Only process *.md files in posts

The editor may leave behind backup files or worse, have a swap file in
the directory due to the post file being edited.  shonku currently
processes it and may even generate a separate post for it under some
conditions.  This patch fixes that by making shonku only process *.md
files and have rsync only sync *.md files in the posts directory.

Signed-off-by: Siddhesh Poyarekar <siddhesh.poyarekar@gmail.com>
Slightly better error messages

I found these useful when debugging for my initial setup.  They're not
perfect, but they're definitely better than what exists now since the
current error messages give no context of where the error occurred.

Signed-off-by: Siddhesh Poyarekar <siddhesh.poyarekar@gmail.com>
Merge pull request #13 from siddhesh/master

Only process *.md files in posts and nice error messages.

Thank you Siddhesh :)