Enabler should report exceptions

[tgiuli]

This came up during one of the past OpenXC workshops. Someone tried to play back a tracefile that was still gzipped, so the Enabler threw a bunch of exceptions in the log, but there was no feedback to the user.

The main screen of the Enabler should show some kind of information about exceptions being thrown. The UI shouldn't badger the user or continuously display scrolling text that is too fast to read. It should also report any kind of problem it has parsing messages, whether from a tracefile or a live data stream.

[peplin]

This would be awesome - Notifications might be a good avenue, so you'll still notice that something is broken even if you don't have the Enabler open.

[mjohn706]

Certainly there needs to be feedback in a case like this. Depending how severe the parse error is, we may want to simply give up on the file completely, which would have been the right thing to do in the gzipped file case. But neither do we want to give up on occasional parse failures. Neither do we want to overwhelm the user with popups, so temporary Notifications are an attractive proposal.

We do need to be careful what exceptions we respond to, though, since recent releases of Android show many exceptions in Logcat even when all is running normally. So we need to be selective about which exceptions we report, such as, our own exceptions, or exceptions resulting from the JSON parse of the tracefile.

[peplin]

This is related to #14

[peplin]

This will happen when #92 is merged - I will create another issue to make sure the trace data source shows an error if it can't playback the file.