[Code health] Add RX utility for handling errors and displaying them in the UI

[scolsen]

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@gino-m proposed adding an RX converter called handleErrors that would effectively serve the role of doOnError, onErrorReturn, and friends and do the following:

1. Catch an error emitted by a stream.
2. Log an exception
3. (if applicable) show an error message in the UI in a consistent manner.

It'd be used (roughly) as follows:

offlineBaseMapRepository
            offlineBaseMapRepository.getOfflineAreasOnceAndStream())	            
              .getOfflineAreasOnceAndStream()
              .compose(handleErrors("Error loading base map", ImmutableList.of()));

See Gino's original comment for more info.

[gino-m]

Similarly, we could add a helper called logErrors that reports them to logcat, but doesn't actually eat the error.

[scolsen]

We likely won't need this after we migrate to coroutines.