Database.find_message() used to be able to reliably indicate whether a
message exists or not (in which case it returns None). However, the
recent API change of the notmuch library means we will return None
even for all Xapian exceptions, which happens e.g. when the current
Database has been modified by another project. Therefore the return
value of None cannot be reliably be used to indicate whether a message
exists or not. Make the docs state that explicitely.
Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
