When I introduce area and map user data storage I also revised the room
user data code - specifically TLuaInterpreter::getRoomUserData(...) to
provide informative error messages on various failures, unintentionally
though, this introduced an incompatibility with the documented behaviour
for this function in that, as of commit-79a8905c it was expected to return
an empty string if the given room id (first argument) number did not exist
or the data key (second argument) string was not found for that room.

As it happens the past and current code DOES allow an empty string to be
stored against a given key but the past code did not allow the user to
differentiate it from the case where the room didn't exist (though that
could be found out with other code) or the case where the key was not
present - no workaround.

This commit adds an optional third argument (bool) enableFullErrorReporting
that enables a nil + error message if provided and is true, but which is
false otherwise so that previous scripts retain their expected results from
using the two-argument usage.

Obviously new scripts should use the third option and set it to be true but
they may wish to check for the availability of getAreaUserData() as an
indication that they will get a nil response if the room does not exist or
does not contain the requested key if they anticipate being used on older
version of Mudlet with the non-ideal getRoomUserData() code.

Signed-off-by: Stephen Lyons <slysven@virginmedia.com>