[IndexedDB] add "dataLossMessage" field to upgradeneeded event

As discussed in [thread] we implemented a dataLoss field on the
upgradeneeded event, tracked in [bug]. Following developer feedback on the
implementation in Chrome, we'd like to add a complementary dataLossMessage
field. The combination would act similarly to DOMException/DOMError's name
and message fields. dataLoss provides something actionable to script (and
the range of values will be well defined in a future iteration of the spec)
and dataLossMessage provides a diagnostic message available for logging and
to the script author during development. Some example messages would
be "missing files", "log record too small", "corrupted internal key", and
"partial record".

In addition to being more transparent to web developers, this will
allow developers to file better bugs against implementations. For example,
Google Docs could log the messages and notice the upward trend of a
specific error if a bug is introduced to the browser implementation.

[thread]
http://lists.w3.org/Archives/Public/public-webapps/2013JanMar/0355.html
[bug] https://www.w3.org/Bugs/Public/show_bug.cgi?id=22370