mon: Monitor: clearer output on error during attempt to convert store

This makes the most obvious cases, that shouldn't do any harm, to output clearer messages.

However, we kept a whole bunch of asserts all over the store convert procedure.  We want to be strict with this, and let the user know something terrible happened during the process so they come running to us, telling us what happened.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>