Define appropriate exception subclass for JNI errors

[aaaaalbert]

The JNI code/glue should not raise generic Exceptions as done here,

https://github.com/aaaaalbert/sensibility-testbed/blob/use-pure-gradle/app/src/main/jni/jniglue.c#L426

but instead define its own exception subclass. This may happen in C or Python, whatever suits us better.

[lukpueh]

Do you have something particular in mind? Should we provide a generic SensibilityError, or one per *Service.java?

[aaaaalbert]

@lukpueh, not really. (For comparison, I'm working around the permissions issue in SensibilityTestbed/repy_v2#4 by hiding the Java SecurityException and returning a JSONified null to the caller.)