Unconditionally activate the occur stuff in ledger-occur-mode

Well, we still deactivate it when regex is nil, but the function
should not look at previous value of ledger-occur-mode:

- the interactive function (ledger-occur) already do it, we don't need
  to do it there,
- caller that want to deactivate the occur stuff only have to call
  ledger-occur with a nil regex
- the old behavior make ledger-reconcile to turn off occur stuff if it
  was already turn on, when what we do want is that the occur stuff
  change to the new account.
In ledger-occur, hide nothing if there is no match

This could cause error when reconciling transaction that are included.

Some message should be shown to explain why nothing happen when
interactively call ledger-occur.
Merge pull request #152 from vanicat/t/occur-bugs

T/occur bugs