Add the ability to retrieve the last error that occured on a device

[rbino]

Right now Data Updater Plant detects if there's something wrong with a specific device (e.g. wrong data type, missing interface etc) and reacts by logging an error and disconnecting the device.

#135 also added support for triggers containing the device errors, but these can be seen only if they're directed to an external backend or if the dashboard is open while the error is happening.

We could, instead, also store in the database details about the last error occured on a specific device. This way operators can debug a device after the fact and have some details about what went wrong. This could be a single entry or (potentially) a log with a reasonably short TTL to provide additional context.