SQLite db:drop:all tiny fix + more teeny tinies

Hi all,

Just some tiny fixes looking for reviews before the new year:

http://dev.rubyonrails.org/ticket/10577 - this one is for the
db:drop:all task, so that it doesn't silently ignore errors removing
SQLite databases.
http://dev.rubyonrails.org/ticket/10621 - docfix for incorrect example
for assert_valid_keys
http://dev.rubyonrails.org/ticket/10631 - another tiny cleanup to the
db:sessions:clear rake task

Thanks for your time again!

Chu Yeow